go.toolchain.rev: update to Go 1.20rc3

Updates #7123

Change-Id: Ibdf53530251c120e7e20c24abcf4a05f2ff7ac97
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f