feat: validate --use-last-valid-config-for-fallback flag

[czeslavo]

What this PR does / why we need it:

Adds validation for the --use-last-valid-config-for-fallback flag that is only relevant when the FallbackConfiguration feature gate is turned on.

Which issue this PR fixes:

Part of #5854.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.6%. Comparing base (74f527d) to head (db9afe3).
Report is 4 commits behind head on main.

❗ Current head db9afe3 differs from pull request most recent head 52b05f3

Please upload reports for the commit 52b05f3 to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6168     +/-   ##
=======================================
- Coverage   73.7%   73.6%   -0.1%     
=======================================
  Files        198     198             
  Lines      19724   19742     +18     
=======================================
- Hits       14547   14546      -1     
- Misses      4196    4213     +17     
- Partials     981     983      +2     

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