Move setup-java to v5 and enable Gradle's build cache

- setup-java v4 is deprecated; use v5 (still with `cache: gradle`,
  which works once job containers can reach the runner's cache server).
- Enable Gradle's build cache (org.gradle.caching) so unchanged
  compilations are restored from cache instead of re-run.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.gitea/workflows/check.yml

@@ -17,8 +17,11 @@ jobs:
         with:
           submodules: true
 
+      # The JDK comes from the runner's tool cache. `cache: gradle` stores Gradle's
+      # dependencies, Loom's Minecraft cache and the build cache in the runner's
+      # actions cache.
       - name: Set up Java 21
-        uses: actions/setup-java@v4
+        uses: actions/setup-java@v5
         with:
           distribution: temurin
           java-version: 21

.gitea/workflows/release.yml

@@ -44,8 +44,11 @@ jobs:
           fi
           echo "Building Saturn Client $MOD_VERSION"
 
+      # The JDK comes from the runner's tool cache. `cache: gradle` stores Gradle's
+      # dependencies, Loom's Minecraft cache and the build cache in the runner's
+      # actions cache.
       - name: Set up Java 21
-        uses: actions/setup-java@v4
+        uses: actions/setup-java@v5
         with:
           distribution: temurin
           java-version: 21