gopls: update to 0.9.4.

This release fixes an unfortunate bug in a new vet analysis in the
gopls@v0.9.2 release. Specifically, a stray print statement in a
new analyzer for the invalid time format string "2006-02-01", which
corrupts gopls' communication over STDIN/STDOUT.

This only affects projects using that bad format string, and only
when a file in the affected package is open. Thanks to @Phadin for
accurately identifying this bug so quickly after it was introduced.
Issue golang/go#54459 tracks our follow-up to prevent similar
regressions from making it into future gopls releases.

On a positive note, here is the new vet analysis in action. Clearly
it will catch real bugs!

(wiz)