Build/Test Tools: Trim the 6.0 workflows.

This removes reusable workflows that are no longer referenced in the branch, and trims the unit testing matrix to the highest and lowest supported version of each major PHP branch and database version.

Props desrosj, johnbillion

See #64083


git-svn-id: https://develop.svn.wordpress.org/branches/6.0@61810 602fd350-edb4-49c9-b593-d223f7449a82

Author: johnbillion

.github/workflows/phpunit-tests.yml

@@ -46,7 +46,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-latest ]
-        php: [ '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1' ]
+        php: [ '5.6', '7.0', '7.4', '8.0', '8.1' ]
         db-type: [ 'mysql' ]
         db-version: [ '5.7' ]
         multisite: [ false, true ]