project.config: add mdb/google-cla-gerrit-robot-accounts group for SignCLA

Add the mdb/google-cla-gerrit-robot-accounts group, and consider it
to have accepted the "Google CLA" contributor agreement. This change
implements the new Gerrit process for allowlisting bots to bypass
CLA checks via an mdb group, as documented at go/gob/team/signcla.

We don't have bots that make use of this ability yet, but it is safe
to add preemptively, making it possible to start using the mdb group
for future bots or to convert existing ones. It brings Go project's
configuration closer to what can be considered as the default.

Updates go.dev/issue/38075.
Updates b/198293761.

Change-Id: I53792f6786156349fa746441b1b41de7f5e18811