Bump ktfmt to 0.21 and add support to Google and Kotlinlang formats

[JavierSegoviaCordoba]

Manually tested, @cgrushko can you take a look at this PR?

Should getCustomFormattingOptions have a better way to manage ktfmt versioning availability? I was thinking about adding to the Style enum a since property to throw errors.

EDIT:

I added the since property so spotless will throw an exception if the user tries to use a ktfmt version that doesn't have a specific format.

[nedtwigg]

• add link to this PR in plugin-gradle/CHANGES.md, and put it under ### Added
• copy this entry also to plugin-maven/CHANGES.md and CHANGES.md

I noted an optional refactor below. Nice to have but not necessary for a merge-and-release.

[JavierSegoviaCordoba]

@nedtwigg has spotless a way to compare two versions so I can delete the Version class I added?

[nedtwigg]

The version check is nice, but needs to be thread-safe.

[nedtwigg]

Still missing an entry in the root CHANGES.md. Once that gets done I think this is ready to merge and release.

[nedtwigg]

Released in plugin-gradle 5.11.0 and plugin-maven 2.9.0