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Pick the PriorityClass with the lowest value of priority in case more than one global default exists

**What this PR does / why we need it**:
Please see the referenced issue.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #59987

**Special notes for your reviewer**:

**Release note**:

```release-note
Priority admission controller picks a global default with the lowest priority value if more than one such default PriorityClass exists.
```

/sig scheduling
cc/ @liggitt