fix: hardcode java-version '17' in samples-java-helidon-v3.yaml

matrix.version was referenced but the version: matrix dimension was
removed during the compute-matrix refactor, causing setup-java to
receive an empty string. Consistent with the v4 fix (hardcoded '21').

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Picazsoo

.github/workflows/samples-java-helidon-v3.yaml

@@ -48,7 +48,7 @@ jobs:
       - uses: actions/setup-java@v5
         with:
           distribution: 'temurin'
-          java-version: ${{ matrix.version }}
+          java-version: '17'
       - name: Cache maven dependencies
         uses: actions/cache@v5
         env: