Rename :skipped reason to :backoff_in_effect for clarity

What does this MR do?

Renames the :skipped reason to :backoff_in_effect in Geo::Scheduler::SchedulerWorker to make the logs more descriptive.

Why was this MR needed?

As described in issue 8694, the :skipped reason in geo.log was not clear about why a job was skipped. Since the only reason for should_be_skipped? returning true is the 5-minute backoff delay from GeoBackoffDelay, the new name :backoff_in_effect makes the logs more informative for debugging.

What are the relevant issue numbers?

Closes #8694 (closed)

Does this MR meet the acceptance criteria?

• Added a test for the new behavior
• Changelog entry added