Deprecate findbugs

[tasomaniac]

Depreacte Findbugs support since it is completely removed in Gradle 6.x. After this is merged, I will prepare new release with docs update to make it obvious in the Readme.

Used Gradle's official deprecation support. When run with --warning-mode all, it looks like this. First is output by Gradle, the 2nd by us.

The findbugs plugin has been deprecated. This is scheduled to be removed in Gradle 6.0. Consider using the com.github.spotbugs plugin instead.
        at static_analysis_2u2ywn09w7g7lt00gz4re3daa$_run_closure1.doCall(/Users/sdane/dev/taso/gradle-static-analysis-plugin/sample-multi-module/team-props/static-analysis.gradle:25)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)
Novoda Static Analysis Plugin Findbugs support is deprecated. This is scheduled to be removed in version 2.0 Please use SpotBugs instead. https://github.com/novoda/gradle-static-analysis-plugin/blob/master/docs/tools/spotbugs.md
        at static_analysis_2u2ywn09w7g7lt00gz4re3daa$_run_closure1.doCall(/Users/sdane/dev/taso/gradle-static-analysis-plugin/sample-multi-module/team-props/static-analysis.gradle:25)
        (Run with --stacktrace to get the full stack trace of this deprecation warning.)