Raise the Java baseline from 11 to 21

Compiles and publishes at Java 21 instead of Java 11, and moves both CI
workflows to a JDK 21 runner.

This is a breaking change for consumers: the emitted bytecode goes from class
file version 55 to 65, so anything still building or running on Java 11 or 17
can no longer use these artifacts.

Verified: clean build emits major version 65 for both modules with 300 tests
passing, and CFLint - the main downstream consumer - compiles and passes its
675 tests against the result.

CFLint does need a matching change. Its CI pins JDK 11 in gradle.yml and
publish.yml, and loading class file version 65 on an 11 JVM fails with
UnsupportedClassVersionError, so those must move to 21 before it can build
against this.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GzpZFd4rnE1Yi2sVHAji35

Author: claude

.github/workflows/gradle.yml

@@ -17,11 +17,11 @@ jobs:
     - name: Checkout
       uses: actions/checkout@v4
 
-    - name: Set up JDK 11
+    - name: Set up JDK 21
       uses: actions/setup-java@v4
       with:
         distribution: temurin
-        java-version: 11
+        java-version: 21
 
     - name: Setup Gradle
       uses: gradle/actions/setup-gradle@v3

.github/workflows/maven-publish.yml

@@ -23,10 +23,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
-    - name: Set up JDK 11
+    - name: Set up JDK 21
       uses: actions/setup-java@v4
       with:
-        java-version: '11'
+        java-version: '21'
         distribution: 'temurin'
         server-id: github # Value of the distributionManagement/repository/id field of the pom.xml
         settings-path: ${{ github.workspace }} # location for the settings.xml file

build.gradle

@@ -132,8 +132,8 @@ subprojects {
 		copyTo 'META-INF/MANIFEST.MF'
 	}
 
-	sourceCompatibility = 11
-	targetCompatibility = 11
+	sourceCompatibility = 21
+	targetCompatibility = 21
 	tasks.withType(JavaCompile) { options.encoding = 'UTF-8' }
 
 	task sourcesJar(type: Jar) {

pom.xml

@@ -57,7 +57,7 @@
 	<properties>
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 
-		<maven.compiler.release>11</maven.compiler.release>
+		<maven.compiler.release>21</maven.compiler.release>
 
 		<jacoco.version>0.8.12</jacoco.version>
 		<!-- jacoco:prepare-agent overwrites this at build time with the -javaagent setting.