Update object cache test matrix

SQlite Object Cache Plugin requires WP 5.5+.

Follow-up to #227.

Author: swissspidy

.github/workflows/reusable-testing.yml

@@ -295,12 +295,6 @@ jobs:
               "wp": "trunk",
               "dbtype": "sqlite"
             },
-            {
-              "php": "7.2",
-              "wp": "4.9",
-              "dbtype": "sqlite",
-              "object_cache": "sqlite"
-            },
             {
               "php": "8.5",
               "wp": "latest",