Disable Spotless format checking for some generated files. (google#2559)

Files generated by the protobuf compiler are not going to conform to the
expected format. It's not useful to check them.

pom.xml

@@ -187,11 +187,13 @@
  </formats>
 
  <java>
- <!-- Exclude classes which need Java 17 for compilation; Google Java Format internally relies on javac,
- so formatting will fail if build is executed with JDK 11 -->
  <excludes>
+ <!-- Exclude classes which need Java 17 for compilation; Google Java Format internally relies on javac,
+ so formatting will fail if build is executed with JDK 11 -->
  <exclude>src/test/java/com/google/gson/functional/Java17RecordTest.java</exclude>
  <exclude>src/test/java/com/google/gson/native_test/Java17RecordReflectionTest.java</exclude>
+ <!-- Exclude protobuf generated code. -->
+ <exclude>target/generated-test-sources/protobuf/**/*.java</exclude>
  </excludes>
  <googleJavaFormat>
  <style>GOOGLE</style>