Add new Performance/ConcurrentMonotonicTime cop

[koic]

Follow up to rails/rails#43502.

This PR adds new Performance/ConcurrentMonotonicTime cop.

This cop identifies places where Concurrent.monotonic_time can be replaced by Process.clock_gettime(Process::CLOCK_MONOTONIC).

# bad
Concurrent.monotonic_time

# good
Process.clock_gettime(Process::CLOCK_MONOTONIC)

Below is a benchmark.

% cat monotonic_time.rb
#!/usr/local/bin/ruby

p `ruby -v`

require 'concurrent'
require 'benchmark/ips'

Benchmark.ips do |x|
  x.report('Concurrent.monotonic_time') { Concurrent.monotonic_time }
  x.report('Process.clock_gettime')     { Process.clock_gettime(Process::CLOCK_MONOTONIC) }

  x.compare!
end

% ruby monotonic_time.rb
"ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-darwin19]\n"
Warming up --------------------------------------
Concurrent.monotonic_time
                       607.064k i/100ms
Process.clock_gettime
                       730.862k i/100ms
Calculating -------------------------------------
Concurrent.monotonic_time
                          5.695M (± 4.4%) i/s -     28.532M in
                          5.019798s
Process.clock_gettime
                          7.398M (± 2.5%) i/s -     37.274M in
                          5.041776s

Comparison:
Process.clock_gettime:  7397700.9 i/s
Concurrent.monotonic_time:  5695385.0 i/s - 1.30x  (± 0.00) slower

And this cop is compatible with ruby-concurrency/concurrent-ruby#915.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.