chore: Move maven config up

Author: lqiu96

gapic-generator-java-pom-parent/pom.xml

@@ -22,6 +22,7 @@
   <properties>
     <skipUnitTests>false</skipUnitTests>
     <checkstyle.header.file>java.header</checkstyle.header.file>
+    <maven.compiler.release>8</maven.compiler.release>
 
     <!-- External dependencies, especially gRPC and Protobuf version, should be
         consistent across modules in this repository -->
@@ -33,7 +34,6 @@
     <guava.version>33.4.0-jre</guava.version>
     <protobuf-java.version>3.25.8</protobuf-java.version>
     <opentelemetry-bom.version>1.47.0</opentelemetry-bom.version>
-    <maven.compiler.release>8</maven.compiler.release>
     <error_prone_annotations.version>2.38.0</error_prone_annotations.version>
     <j2objc-annotations.version>3.0.0</j2objc-annotations.version>
     <threetenbp.version>1.7.0</threetenbp.version>