chore: Add Java 21 to build and test DEV-2769 (#99)

Build and test project with Java 21 in parallel with Java 17.

Currently temurin [is publishing their Java 21 for production
use](adoptium/temurin#8).

_Out of scope_
Use of Java 21 in published Docker artefact, this is still on Java 17.

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
     name: Build and Run Tests
     strategy:
       matrix:
-        java-version: [ 17 ]
+        java-version: [ 17, 21 ]
     runs-on: ubuntu-latest
     steps:
       - name: Checkout current branch