x/tools/gopls: allow to clear upgrade check diagnostics

[hyangah]

"Check for upgrades", "Upgrade transitive dependencies", "Upgrade direct dependencies" codelenses in go.mod file provide convenient ways of checking available upgrades from time to time.

However, often times, I want to stay with the current dependencies after selectively upgrading a couple of chosen modules.
Currently there is no convenient way to hide the diagnostics produced as a result of running one of these code lenses without upgrading the modules. (except restarting the language server)

It would be nice if there is a way to clear these diagnostics.
One way to do it is to use the approach gc_details codelens uses (toggle).
Another way is to provide a separate codelens that disposes all the upgrade check diagnostics. (One concern is currently, there are too many codelenses :-))