GH Actions: fix up the test matrix

WP 6.6 dropped support for PHP 7.0 and 7.1, so test runs for those PHP versions can no longer run against WP `latest`, but need to be limited to max WP 6.5.

.github/workflows/test.yml

@@ -19,7 +19,7 @@ jobs:
       matrix:
         # Notes regarding supported versions in WP:
         # The base matrix only contains the PHP versions which are supported on all supported WP versions.
-        php: ['7.0', '7.1', '7.2', '7.3']
+        php: ['7.2', '7.3']
         wp: ['latest', '5.2']
         experimental: [false]
 
@@ -69,6 +69,20 @@ jobs:
           - php: '5.6'
             wp: '5.2'
             experimental: false
+          # PHP 7.0 was supported up to WP 6.5.
+          - php: '7.0'
+            wp: '5.2'
+            experimental: false
+          - php: '7.0'
+            wp: '6.5'
+            experimental: false
+          # PHP 7.1 was supported up to WP 6.5.
+          - php: '7.1'
+            wp: '5.2'
+            experimental: false
+          - php: '7.1'
+            wp: '6.5'
+            experimental: false
 
           # Complement the builds run via the matrix with some additional builds against specific WP versions.
           - php: '7.3'