Add support for ktlint 0.50.0

[JakeWharton]

We all know the drill!

Another release, another set of breaking changes.

Thank you for the continued efforts in supporting this tool whose API instability is very painful to endure.

[TWiStErRob]

@nedtwigg do you know when the next release of spotless will come out? (including the above commit)

[nedtwigg]

Released in plugin-gradle 6.20.0 and plugin-maven 2.38.0.