Replace FindBugs with SpotBugs

[mpeng3]

Our static analysis plugin is using the version of FindBugs that's bundled with Gradle (prior to version 6). I have found that a project that is using Gradle 6 or greater cannot use this plugin because the FindBugs plugin is no longer there. Additionally, following the guides on FindBugs' page to import it doesn't seem to be enough, as the FindBugsPlugin, FindBugsExtension, and others don't seem to be included in the standard distribution of FindBugs.

Furthermore, FindBugs has now been deprecated in favor of SpotBugs.

Therefore, I believe we should try to migrate away from FindBugs and convert this library to use SpotBugs instead.