ci: add Java 21 to CI matrix

Author: Lucas McDonald

.github/workflows/ci-workflow.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       fail-fast: false # TODO: Set to true once CI is stable
       matrix:
-        version: [ 8, 11, 17 ]
+        version: [ 8, 11, 17, 21 ]
         distribution: [ corretto, temurin ] # TODO: Add OpenJDK
     uses: ./.github/workflows/build.yml
     secrets: inherit