go.mod, README.md: use Go 1.22

Updates #11058

Change-Id: I95eecdc7afe2b5f8189016fdb8a773f78e9f5c42
Signed-off-by: Brad Fitzpatrick <bradfitz@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