chore: raise minimum required JDK to 21

Author: cortlepp

.github/workflows/ci.yml

@@ -29,7 +29,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: [ 17, 21, 25 ]
+        java: [ 21, 25 ]
     name: "Java ${{ matrix.java }}"
     runs-on: ubuntu-24.04
     steps: