Centralize CI build JDK, update with Renovate

.github/renovate.json5

@@ -0,0 +1,26 @@
+{
+  $schema: 'https://docs.renovatebot.com/renovate-schema.json',
+  extends: [
+    'config:recommended',
+  ],
+  ignorePresets: [
+    // Ensure we get the latest version and are not pinned to old versions.
+    'workarounds:javaLTSVersions',
+  ],
+  customManagers: [
+    // Update .java-version file with the latest JDK version.
+    {
+      customType: 'regex',
+      fileMatch: [
+        '\\.java-version$',
+      ],
+      matchStrings: [
+        '(?<currentValue>.*)\\n',
+      ],
+      datasourceTemplate: 'java-version',
+      depNameTemplate: 'java',
+      // Only write the major version.
+      extractVersionTemplate: '^(?<version>\\d+)',
+    },
+  ]
+}

.github/workflows/.java-version

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

.github/workflows/build.yml

@@ -16,12 +16,10 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Configure JDK
-        uses: actions/setup-java@v4
+      - uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: '21'
-          cache: 'gradle'
+          java-version-file: .github/workflows/.java-version
 
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v4