Update CI/build infra (#1852)

* Update CI/build infra

- Gradle caching on CI
- JDK 21
- Move gradle wrapper validation to a separate workflow that only runs on changes to those files

* Update .github/workflows/gradle-wrapper.yml

Co-authored-by: Jake Wharton <jw@squareup.com>

---------

Co-authored-by: Jake Wharton <jw@squareup.com>

.github/workflows/build.yml

@@ -16,16 +16,16 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Validate Gradle Wrapper
-        uses: gradle/actions/wrapper-validation@v3
-
       - name: Configure JDK
         uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: '20'
+          java-version: '21'
           cache: 'gradle'
 
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v3
+
       - name: Test
         run: ./gradlew build check --stacktrace -PkotlinTestMode=${{ matrix.kotlin-test-mode }}