Coding Standards: Remove the PHP Compatibility scanning job from the `allowed_failures` list.

All pre-existing compatibility warnings and errors as flagged by the PHPCompatibilityWP ruleset have been addressed.

Fixes #49922.
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git-svn-id: https://core.svn.wordpress.org/trunk@47813 1a063a9b-81f0-0310-95a4-ce76da25c4cd