Make priorityClassSource in the SchedulePriority of PropagationPolicy a required field

[seanlaii]

What type of PR is this?
/kind api-change
/kind feature

What this PR does / why we need it:
Making it a required field allows for more flexibility in future changes. For example, if we need to modify the default value later, we can make it optional and explicitly define a default value.

Which issue(s) this PR fixes:
Part of #5961

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Make priorityClassSource in the SchedulePriority of PropagationPolicy a required field.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.11%. Comparing base (7b29afb) to head (ac22188).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6163   +/-   ##
=======================================
  Coverage   48.11%   48.11%           
=======================================
  Files         668      668           
  Lines       55344    55344           
=======================================
+ Hits        26626    26629    +3     
+ Misses      26982    26980    -2     
+ Partials     1736     1735    -1     

Flag	Coverage Δ
unittests	48.11% <ø> (+<0.01%)	⬆️

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[RainbowMango]

/lgtm
/approve
Thanks.

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