x/tools/gopls: reset diagnostics when toggling vulncheck on/off

[findleyr]

Noticed by @hyangah: when toggling vulncheck off, diagnostics do not clear until the user starts typing.

This is yet another case of #42814, but that is too risky a change to make right now.

For this release, a targeted fix.

CC @adonovan

[gopherbot]

Change https://go.dev/cl/498560 mentions this issue: gopls/internal/lsp: clear vuln diagnostics on config changes