GHA gradle.yml: use java 21

Author: msgilligan

.github/workflows/gradle.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macOS-latest, windows-latest]
-        java: ['17']
+        java: ['21']
         distribution: ['temurin']
       fail-fast: false
     name: ${{ matrix.os }} JDK ${{ matrix.java }}