Revert "Use JRE 25 in CI jobs (#854)"

This reverts commit a893925.

Author: c-dilks

.github/workflows/ci.yml

@@ -21,8 +21,9 @@ defaults:
     shell: bash
 
 env:
-  java_version: 25
+  java_version: 21
   java_distribution: zulu
+  javadoc_version: 24 # newer than `java_version` for better javadoc
   groovy_version: 4.x
   CCDB_CONNECTION: 'sqlite:////cvmfs/oasis.opensciencegrid.org/jlab/hallb/clas12/sw/noarch/data/ccdb/ccdb_latest.sqlite'
   nthreads: 1
@@ -342,7 +343,7 @@ jobs:
       - name: set up JDK
         uses: actions/setup-java@v5
         with:
-          java-version: ${{ env.java_version }}
+          java-version: ${{ env.javadoc_version }}
           distribution: ${{ env.java_distribution }}
           cache: maven
       - name: build coatjava javadocs # javadoc:aggregate output dir cannot be controlled, so assume the latest "standard" path and `mv` it