Upgrade to use Java 21

The upgrade concerns both the compilation and the CI.

Author: kostis

.github/workflows/ci.yml

@@ -10,10 +10,10 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v5
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v5
         with:
-          java-version: '17'
           distribution: 'temurin'
+          java-version: '21'
       - name: Compile, check, and run tests
         run: mvn verify assembly:single

pom.xml

@@ -12,8 +12,8 @@
 
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <maven.compiler.source>17</maven.compiler.source>
-        <maven.compiler.target>17</maven.compiler.target>
+        <maven.compiler.source>21</maven.compiler.source>
+        <maven.compiler.target>21</maven.compiler.target>
 
         <!-- Dependency versions appear alphabetically below -->
         <checker-qual.version>3.52.0</checker-qual.version>
@@ -105,11 +105,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>${compiler-plugin.version}</version>
                 <configuration>
-                    <release>17</release>
+                    <release>21</release>
                     <showWarnings>true</showWarnings>
-                    <compilerArgs>
-                         <arg>--enable-preview</arg>
-                    </compilerArgs>
                 </configuration>
             </plugin>