Build/Test Tools: Trim the 5.4 workflows.

This trims the unit testing matrix to the highest and lowest supported version of each major PHP branch.

Props desrosj, johnbillion

See #64083


git-svn-id: https://develop.svn.wordpress.org/branches/5.4@61816 602fd350-edb4-49c9-b593-d223f7449a82

Author: johnbillion

.github/workflows/phpunit-tests.yml

@@ -43,7 +43,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-latest ]
-        php: [ '5.6', '7.0', '7.1', '7.2', '7.3', '7.4' ]
+        php: [ '5.6', '7.0', '7.4' ]
         multisite: [ false, true ]
         split_slow: [ false ]
         memcached: [ false ]