ci: build against LTS Java versions (11, 17, 21, 25)

Temurin doesn't publish JDK 15, and it's an EOL non-LTS release anyway.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: oryan-block

.github/workflows/pull-request.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ '11', '15', '17' ]
+        java: [ '11', '17', '21', '25' ]
     steps:
       - name: Checkout
         uses: actions/checkout@v7