Update: target JDK to 21 as reference;

Author: olanokhin

.github/workflows/pull-request.yml

@@ -12,7 +12,7 @@ jobs:
       - uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: '17'
+          java-version: '21'
       - name: Lint
         run: |
           ./gradlew ktlintCheck