x/tools/gopls: migrate to jsonrpc2_v2

[findleyr]

The new version of the jsonrpc2 package (authored by @ianthehat) has improved APIs and shutdown semantics. @bcmills has also recently worked on improving its concurrency model.

This issue tracks work to do the necessary refactoring to migrate to the new APIs.

[bcmills]

FWIW, my stack for the concurrency model is still pending. (I was hoping to make progress on it during the Quiet Week last week, but didn't end up with enough bandwidth.)

[bcmills]

jsonrpc2_v2 should be stable at this point, whereas the old jsonrpc2 is at least a little flaky (#55179). We should do this migration at some point in the not too distant future.