Improve _update_bundle.yaml to sign commits for newly created PRs

[taurus-forever]

Hi,

We have recently enabled canonical repo automation on all Data repos and it enforced requirements for the signed commits.
At the moment the update-bundle workflow doesn't not sign commit. It is a issue to add such possubility:

STR:
Create a new PR using _update_bundle.yaml

AR:
canonical/postgresql-k8s-bundle#509 cannot be merged due to Commits must have verified signatures .

ER:
Newly created PR has all commits signed by Canonical GH bot SSH key.

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/DPE-9060.

This message was autogenerated

[carlcsaposs-canonical]

I think that might violate our security policies (sent details privately)