main.star: disable FreeBSD 14.1 on old release branches

We're not planning to backport the fixes needed to make it work.

For golang/go#61095.

Change-Id: I6dfb039d0bec335e26332aef7ace17691609011d
Reviewed-on: https://go-review.googlesource.com/c/build/+/634079
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: mknyszek