Bump Rust version (MSRV) (sigp#4204)

There was a [`VecDeque` bug](rust-lang/rust#108453) in some recent versions of the Rust standard library (1.67.0 & 1.67.1) that could cause Lighthouse to panic (reported by `@Sea Monkey` on discord). See full logs below.

The issue was likely introduced in Rust 1.67.0 and [fixed](rust-lang/rust#108475) in 1.68, and we were able to reproduce the panic ourselves using [@michaelsproul's fuzz tests](https://github.com/michaelsproul/lighthouse/blob/fuzz-lru-time-cache/beacon_node/lighthouse_network/src/peer_manager/fuzz.rs#L111) on both Rust 1.67.0 and 1.67.1.

Users that uses our Docker images or binaries are unlikely affected, as our Docker images were built with `1.66`, and latest binaries were built with latest stable (`1.68.2`). It likely impacts user that builds from source using Rust versions 1.67.x.

Bump Rust version (MSRV) to latest stable `1.68.2`.

From `@Sea Monkey` on Lighthouse Discord:

> Crash on goerli using `unstable` `dd124b2d6804d02e4e221f29387a56775acccd08`

```
thread 'tokio-runtime-worker' panicked at 'Key must exist', /mnt/goerli/goerli/lighthouse/common/lru_cache/src/time.rs:68:28
stack backtrace:
Apr 15 09:37:36.993 WARN Peer sent invalid block in single block lookup, peer_id: 16Uiu2HAm6ZuyJpVpR6y51X4Enbp8EhRBqGycQsDMPX7e5XfPYznG, error: WouldRevertFinalizedSlot { block_slot: Slot(5420212), finalized_slot: Slot(5420224) }, root: 0x10f6…3165, service: sync
   0: rust_begin_unwind
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:64:14
   2: core::panicking::panic_display
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:135:5
   3: core::panicking::panic_str
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/panicking.rs:119:5
   4: core::option::expect_failed
             at /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/core/src/option.rs:1879:5
   5: lru_cache::time::LRUTimeCache<Key>::raw_remove
   6: lighthouse_network::peer_manager::PeerManager<TSpec>::handle_ban_operation
   7: lighthouse_network::peer_manager::PeerManager<TSpec>::handle_score_action
   8: lighthouse_network::peer_manager::PeerManager<TSpec>::report_peer
   9: network::service::NetworkService<T>::spawn_service::{{closure}}
  10: <futures_util::future::select::Select<A,B> as core::future::future::Future>::poll
  11: <futures_util::future::future::map::Map<Fut,F> as core::future::future::Future>::poll
  12: <futures_util::future::future::flatten::Flatten<Fut,<Fut as core::future::future::Future>::Output> as core::future::future::Future>::poll
  13: tokio::loom::std::unsafe_cell::UnsafeCell<T>::with_mut
  14: tokio::runtime::task::core::Core<T,S>::poll
  15: tokio::runtime::task::harness::Harness<T,S>::poll
  16: tokio::runtime::scheduler::multi_thread::worker::Context::run_task
  17: tokio::runtime::scheduler::multi_thread::worker::Context::run
  18: tokio::macros::scoped_tls::ScopedKey<T>::set
  19: tokio::runtime::scheduler::multi_thread::worker::run
  20: tokio::loom::std::unsafe_cell::UnsafeCell<T>::with_mut
  21: tokio::runtime::task::core::Core<T,S>::poll
  22: tokio::runtime::task::harness::Harness<T,S>::poll
  23: tokio::runtime::blocking::pool::Inner::run
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
Apr 15 09:37:37.069 INFO Saved DHT state                         service: network
Apr 15 09:37:37.070 INFO Network service shutdown                service: network
Apr 15 09:37:37.132 CRIT Task panic. This is a bug!              advice: Please check above for a backtrace and notify the developers, message: <none>, task_name: network
Apr 15 09:37:37.132 INFO Internal shutdown received              reason: Panic (fatal error)
Apr 15 09:37:37.133 INFO Shutting down..                         reason: Failure("Panic (fatal error)")
Apr 15 09:37:37.135 WARN Unable to free worker                   error: channel closed, msg: did not free worker, shutdown may be underway
Apr 15 09:37:39.350 INFO Saved beacon chain to disk              service: beacon
Panic (fatal error)
```

.github/workflows/test-suite.yml

@@ -13,7 +13,7 @@ env:
   # Disable debug info (see https://github.com/sigp/lighthouse/issues/4005)
   RUSTFLAGS: "-D warnings -C debuginfo=0"
   # The Nightly version used for cargo-udeps, might need updating from time to time.
-  PINNED_NIGHTLY: nightly-2022-12-15
+  PINNED_NIGHTLY: nightly-2023-04-16
   # Prevent Github API rate limiting.
   LIGHTHOUSE_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 jobs:

lighthouse/Cargo.toml

@@ -4,7 +4,7 @@ version = "4.0.1"
 authors = ["Sigma Prime <[email protected]>"]
 edition = "2021"
 autotests = false
-rust-version = "1.66"
+rust-version = "1.68.2"
 
 [features]
 default = ["slasher-mdbx"]