Reduce the PHP test matrix to the latest versions of PHP 5, 7 and 8

This should be sufficient to find all version-dependent errors. Mind
the global warming.

Author: alexander-nitsche

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        php: [ '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2' ]
+        php: [ '5.6', '7.4', '8.2' ]
 
     steps:
       - name: Checkout repository