[BUGS-7429] Add filter to exclude select post types from purge

[jazzsequence]

Update the Purger class to add a filter that allows users skip purges if a given post type is being updated.

This PR has been updated to include various CI tweaks.

• we're using wpunit-helpers
• I've broken validate-fixture-version.sh into a separate gist with the goal of standardizing that script and making it more robust
• since the filter is a feature add, I've bumped the versions everywhere to 1.5.0-dev

[github-actions]

Composer Changes

Dev Packages	Operation	Base	Target
nikic/php-parser	Upgraded	v5.0.0	v5.0.1
pantheon-systems/wpunit-helpers	New	-	v2.0.0
phpstan/phpdoc-parser	Upgraded	1.25.0	1.26.0
phpunit/phpunit	Upgraded	9.6.16	9.6.17
squizlabs/php_codesniffer	Upgraded	3.8.1	3.9.0

[pwtyler]

This is superfluous if we're going to ship a release immediately, but in general, prefer to use the x.y.z-dev version in documentation and docblocks, then search-replace as part of release prep. It could sit on develop and then subsequent feature bump to a minor/major vn, "@SInCE" will then point to a non-existent version.

if we ever get to implementing, plugin release actions should handle inside php files already.