Make formatter and impsort cache version-specific

[famod]

Avoids inconsistencies like this one: #23944 (comment)

The obvious downside is that on plugin update, obsolete cache files of the older version remain present.
But then again those plugins are updated only rarely. Btw, formatter cache file for core-deployment is ~88k.

[gastaldi]

The obvious downside is that on plugin update, obsolete cache files of the older version remain present.

Maybe we could delete the .cache dir when mvn clean -DdeleteCache is used?

Or always delete it with mvn clean and introduce a -DkeepCache instead

[gsmet]

I think it's not that big of a problem.

[famod]

The obvious downside is that on plugin update, obsolete cache files of the older version remain present.

Maybe we could delete the .cache dir when mvn clean -DdeleteCache is used?

Doable, but same problem as merely documenting the find + rm command: hard to find.

Or always delete it with mvn clean and introduce a -DkeepCache instead

Not removing it via clean was the sole purpose of moving it to .cache outside of target.
quickly is using clean by default and I suppose hardly anyone would use/know that custom flag.

Another possibility would be (on clean) to remove everything in .cache except the folders for the current version
But that's pretty verbose with Maven...