Migrate sonar-findbugs to com.github.spotbugs groupId

[KengoTODA]

Now sonar-findbugs is not maintained by SonarSource, and nobody deploy its artifact to Maven central.
Then we can change its groupId to com.github.spotbugs from version 3.7 and deploy to maven central by us.

In my understanding, we do not need to migrate their Git repo to our organization, but it should be better to rename its name and artifactId to sonar-spotbugs.

refs: spotbugs/sonar-findbugs#135

[KengoTODA]

From 3.7.0, sonar-findbugs has been migrated to com.github.spotbugs groupId.

https://github.com/spotbugs/sonar-findbugs/blob/3.7.0/pom.xml