go.toolchain.branch: update to Go 1.20

Updates #7123

Change-Id: I64f6d8de5bb511a23318118b4ea1146247f1ad7c
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