Fix build under Java 21

javac --source --target are redundant with --release

See also MariaDB4j/MariaDB4j@030ccd5

and e.g. quarkusio/quarkus#23799
and https://stackoverflow.com/q/43102787/421602,
based on https://openjdk.org/jeps/247.

pom.xml

@@ -47,9 +47,7 @@
     <!-- TODO https://github.com/vorburger/ch.vorburger.exec/issues/117
          This is currently inconsistent, we should either fully move to 11, or stick to 8... -->
     <!-- Keep these versions aligned with .github/workflows/maven.yml -->
-    <maven.compiler.source>1.8</maven.compiler.source>
-    <maven.compiler.target>1.8</maven.compiler.target>
-    <maven.compiler.release>8</maven.compiler.release>
+    <maven.compiler.release>11</maven.compiler.release>
     <error-prone.version>2.23.0</error-prone.version>
   </properties>
 
@@ -60,8 +58,6 @@
         <artifactId>maven-compiler-plugin</artifactId>
         <version>3.11.0</version>
         <configuration>
-          <source>${maven.compiler.source}</source>
-          <target>${maven.compiler.target}</target>
           <release>${maven.compiler.release}</release>
           <showWarnings>true</showWarnings>
           <compilerArgs>