fix(build): use releaseJavaVersion for kotlin-maven-plugin jvmTarget (#1874)

kotlin-maven-plugin's <jvmTarget> referenced the undeclared property
maven.compiler.target, which silently resolved to null (Maven doesn't
fail on unresolved plugin-config expressions), so the plugin fell back
to its internal default and compiled Kotlin sources targeting JVM 1.8
instead of the project's actual release target.

Introduced in 740895e.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: asolntsev

pom.xml

@@ -539,7 +539,7 @@
                     </execution>
                 </executions>
                 <configuration>
-                    <jvmTarget>${maven.compiler.target}</jvmTarget>
+                    <jvmTarget>${releaseJavaVersion}</jvmTarget>
                 </configuration>
             </plugin>
             <plugin>