MODWRKFLOW-60: Revert minimum Java back to 21.

The **FOLIO** services are not all ready for **Java** 25.
Switch the minimum version to 21.

This should still work under both 21 and 25.

Author: kaladay

.github/workflows/maven.yml

@@ -17,7 +17,6 @@ jobs:
 
     # Set settings due to this maven project acting as a library or dependency to other FOLIO modules.
     with:
-      java-version: '25'
 
       # The spring-module-core dependency currently only has snapshot releases.
       allow-snapshots-release: true