x/tools/gopls: warn about EOL for Go 1.18 support

[findleyr]

Reminder issue: we should mark Go 1.18 as EOL in the next minor gopls release (v0.15.0).

Support will then be removed for gopls@v0.16.0, which will come out shortly after Go 1.22.

[gopherbot]

Change https://go.dev/cl/549115 mentions this issue: gopls/internal/util/goversion: warn about EOL for Go 1.18

[gopherbot]

Change https://go.dev/cl/567418 mentions this issue: x/tools: drop go1.18 support