go.mod: update to latest certstore

It includes a fix to allow us to use Go 1.18.
We can now remove our Tailscale-only build tags.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f