Upgrade to Revapi 0.21.0 and Maven Revapi 0.11.5

See https://revapi.org/news/20200527-releases.html

The most interesting being

• check maven goal now has ignoreSuggestionsFormat property. Use this property to make the maven plugin print out the suggestions for ignoring the found problems in either JSON (by setting it to json, which is the default) or in XML (by setting it to xml).