all: remove "no 1.18 support" failures

We have worked around the issue in DERP,
so the vanilla Go 1.18 toolchain now works.

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