update Tower to 0.4.13 to fix load balancer panic (#1758)

Tower [v0.4.13] includes a fix for a bug in the `tower::ready_cache`
module, tower-rs/tower#415. The `ready_cache` module is used internally
in Tower's load balancer. This bug resulted in panics in the proxy
(linkerd/linkerd2#8666, linkerd/linkerd2#6086) in cases where the
Destination service sends a very large number of service discovery
updates (see linkerd/linkerd2#8677).

This commit updates the proxy's dependency on `tower` to 0.4.13, to
ensure that this bugfix is picked up.

Fixes linkerd/linkerd2#8666
Fixes linkerd/linkerd2#6086

[v0.4.13]: https://github.com/tower-rs/tower/releases/tag/tower-0.4.13