Add signature "-s" to the upmerge action PR creation

[ytimocin]

Add signature "-s" to the upmerge action PR creation

AB#17878

[ytimocin]

Relevant code in samples repository: https://github.com/radius-project/samples/blob/v0.54/.github/workflows/upmerge.yaml#L69.

[willtsai]

It looks like the --signoff flag is already used in the workflow, this issue will be to investigate why it's not working: https://github.com/radius-project/docs/blob/v0.54/.github/workflows/upmerge.yaml#L78