refactor(ci): use single latest Java version (21) for testing

- Change CI test matrix from Java 17+21 to Java 21 only
- Update env.JAVA_VERSION to 21 for consistency across workflow
- Update Codecov upload condition to match new Java version

This simplifies the template CI by testing only on the latest Java version,
as requested for the template repository.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: konard

.github/workflows/release.yml

@@ -35,7 +35,7 @@ concurrency:
   cancel-in-progress: true
 
 env:
-  JAVA_VERSION: '17'
+  JAVA_VERSION: '21'
   JAVA_DISTRIBUTION: 'temurin'
 
 jobs:
@@ -127,7 +127,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        java: ['17', '21']
+        java: ['21']
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -143,7 +143,7 @@ jobs:
         run: mvn test jacoco:report -B
 
       - name: Upload coverage to Codecov
-        if: matrix.os == 'ubuntu-latest' && matrix.java == '17'
+        if: matrix.os == 'ubuntu-latest' && matrix.java == '21'
         uses: codecov/codecov-action@v4
         with:
           file: target/site/jacoco/jacoco.xml