.github: use JDK 17, 21 in CI (LTS versions supported by gradle)

Author: LunNova

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
         runs-on: ubuntu-latest
         strategy:
             matrix:
-                jdk: [11, 15]
+                jdk: [17, 21]
         steps:
             - uses: actions/checkout@1e31de5234b9f8995739874a8ce0492dc87873e2 # v4.0.0
             - uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
@@ -58,7 +58,7 @@ jobs:
               uses: actions/setup-java@6a0805fcefea3d4657a47ac4c165951e33482018 # v4.2.2
               with:
                   distribution: 'adopt'
-                  java-version: 11
+                  java-version: 17
 
             - name: Build and publish to github
               run: ./gradlew -P releasing publish publishPlugins githubRelease -s