Backout Java 21 from build

Author: PhilWakelin

.github/workflows/java.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        jdk: [8, 11, 17, 21]
+        jdk: [8, 11, 17]
 
     steps:
       - uses: actions/checkout@v3
@@ -34,7 +34,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-         jdk: [8, 11, 17, 21]
+         jdk: [8, 11, 17]
 
     steps:
       - uses: actions/checkout@v3
@@ -53,7 +53,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-         jdk: [8, 11, 17, 21]
+         jdk: [8, 11, 17]
 
     steps:
     - uses: actions/checkout@v3