[Whats new] Configure PR title strings to ignore

[BillWagner]

The set of strings for ignored PRs should be configurable.

There are other repos that have requested to add variations of the following text strings in PR repos:

• "repo_sync_working_branch"
• "main" or "merge release"
• "(Internal)" or "[Internal]" - case insensitive.
• "cruft"

In addition, the "learn-build-service-prod" bot should be excluded.

[BillWagner]

/cc @TerryGLee