main.star: don't test the main branch on macOS 11

Go 1.25 will require macOS 12 Monterey or later.

For golang/go#69839.

Change-Id: I5d81c4ea9a1a267e219eb8023b6c20ac6552f701
Reviewed-on: https://go-review.googlesource.com/c/build/+/646395
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: dmitshur