tailcfg: no-op bump of MapRequest.Version

So 1.18 and 1.20 don't have the same.

Change-Id: Ib2cac7c11eb37d9a0c2fcb66630f1cae619a97f4
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