Add git signoff to the commit in the upmerge workflow

[brooke-hamilton]

Description

This pull request includes a change to the .github/workflows/upmerge.yaml file. The change ensures that commits are signed off when performing an upmerge to the edge branch.

• .github/workflows/upmerge.yaml: Modified the git commit command to include the --signoff flag, ensuring that commits are signed off during the upmerge process.

Git log entries for the commit will now look like this, which satisfies the DCO requirement:

Author: Radius CI Bot <[email protected]>
Date:   Wed Nov 27 14:52:43 2024 +0000

    Upmerge to edge
    
    Signed-off-by: Radius CI Bot <[email protected]>

Issue reference

Fixes: #1884