Move Java version into file (#374)

Author: JakeWharton

.github/.java-version

@@ -0,0 +1 @@
+21

.github/workflows/build.yaml

@@ -21,7 +21,7 @@ jobs:
       - uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: 21
+          java-version-file: .github/.java-version
 
       - run: ./gradlew build dokkaHtml
 

.github/workflows/release.yaml

@@ -17,7 +17,7 @@ jobs:
       - uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: 21
+          java-version-file: .github/.java-version
 
       - name: Build and publish artifacts
         env: