Remove automated builds from the php81 buster image

Upstream images aren't built anymore (they have deprecated
buster and added bookworm), so we don't need to coninuously
be checking php81 versions any more. Part of #179

Also, update its README to show it's not being updated.

Author: stronk7

.github/workflows/trigger_new_builds.yml

@@ -2,7 +2,8 @@
 
 A Moodle PHP environment configured for Moodle development based on [Official PHP Images](https://hub.docker.com/_/php/).
 
-### Version
+### Version <span style="color:darkred">(Not updated since 8.1.20, June 2023!)</span>
+
 
 | PHP Version  | Variant | Tags             | Status |
 |--------------|---------|------------------|--------|