Build/Test Tools: Change the patch version used for testing the 5.6 branch of PHP.

This unpins the version of PHP 5.6.20 used when testing the 5.6 branch of PHP.

Using 5.6.20 was an intentional decision in [49162], as that patch version is the oldest version of PHP 5.6 still supported by WordPress. However, as of September 30, 2021, the `5.6.20` image contains a version of OpenSSL with an expired certificate, sometimes resulting in an expired trust chain.

Props Clorith, hellofromTonya, SergeyBiryukov, desrosj.
Merges [51889] to `trunk`.
Fixes #54223.

git-svn-id: https://develop.svn.wordpress.org/trunk@51890 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/phpunit-tests.yml

@@ -66,19 +66,19 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php: [ '5.6.20', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1' ]
+        php: [ '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1' ]
         os: [ ubuntu-latest ]
         memcached: [ false ]
         split_slow: [ false ]
         multisite: [ false, true ]
         include:
           # Additional "slow" jobs for PHP 5.6.
-          - php: '5.6.20'
+          - php: '5.6'
             os: ubuntu-latest
             memcached: false
             multisite: false
             split_slow: true
-          - php: '5.6.20'
+          - php: '5.6'
             os: ubuntu-latest
             memcached: false
             multisite: true