project.config: add check for Signed-off-by footer

The Go project doesn't use Signed-off-by footers in commit messages.
This is documented at https://go.dev/wiki/CommitMessage. Occasional
contributors will sometimes miss this and include it anyway.

Add a submit requirement that catches accidental uses of such footers
so that this isn't something reviewers need to manually pay attention
to when reviewing CLs.

Change-Id: I0250c6336806ef8eea86fbd88084c615d5f0c54d
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/584938
Reviewed-by: Ian Lance Taylor <iant@google.com>