fix: fully encapsulating rejected execution handling

[shawkins]

Description

throwing RejectedExecutionException was added to the SerialExecutor for consistency between the case when there was a local shutdown vs having the underlying executor shutdown. In every instance since then when this has come up we don't actually care about the work not being performed after shutdown - it's just a timing issue where we haven't check for shutdown prior to submitting the work. Rather than continue to change calling code, it seems better to just change the SerialExecutor contract and handle the rejections there.

closes: #6863

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change
• Chore (non-breaking change which doesn't affect codebase;
  test, version modification, documentation, etc.)

Checklist

• Code contributed by me aligns with current project license: Apache 2.0
• I Added CHANGELOG entry regarding this change
• I have implemented unit tests to cover my changes
• I have added/updated the javadocs and other documentation accordingly
• No new bugs, code smells, etc. in SonarCloud report
• I tested my code in Kubernetes
• I tested my code in OpenShift

[sonarqubecloud]

Quality Gate failed

Failed conditions
71.4% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[shawkins]

Not sure about the flakey test here - nothing jumps out at me, and it does not seem reproducible.

[manusa]

There are a couple of flaky tests that I need to fix, don't worry.

[manusa]

LGTM, thx!