build: pin project to JDK 25 and gradle CI to JDK 21

Author: thisisalexandercook

.github/workflows/ci.yml

@@ -12,12 +12,21 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
 
+      - name: Set up JDK 21 for Gradle to run on
+        uses: actions/setup-java@v4
+        with:
+          java-version: 21
+          distribution: 'temurin'
+
       - name: Set up JDK 25
         uses: actions/setup-java@v4
         with:
           distribution: temurin
           java-version: 25-ea
           cache: gradle
 
+      - name: Inject JAVA_HOME_21_64 into `gradle.properties` to always use JDK 21 for Gradle
+        run: mkdir ~/.gradle && echo "org.gradle.java.home=$JAVA_HOME_21_X64" >> ~/.gradle/gradle.properties
+
       - name: Build with Gradle
         run: ./gradlew build

build.gradle

@@ -7,4 +7,12 @@ subprojects {
     repositories {
 	mavenCentral()
     }
+
+    plugins.withId('java') {
+        java {
+            toolchain {
+                languageVersion = JavaLanguageVersion.of(25)
+            }
+        }
+    }
 }