go.toolchain.rev: bump to Go 1.26.3

Updates tailscale/corp#41490

Change-Id: I35b67bdbcd71468fea03b033b17aeefe1319dc45
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