Configure commit author, signing and signoff in GitHub action for image updates

[SaschaSchwarze0]

Changes

The GitHub action we are using to create a pull request is distinguishing author and committer of the commit it creates. See https://github.com/peter-evans/create-pull-request/tree/v7/?tab=readme-ov-file#action-inputs

In #1692, the author is somehow the openshift-merge-robot.

Based on the action documentation, the author is taken from github.actor.

Based on https://github.com/orgs/community/discussions/25067, github.actor for a workflow run triggered by a schedule is the latest user that modified the schedule trigger. In our case that is the openshift-merge-robot because it merges our PRs including the PR that introduced or modified the GitHub action.

I am hereby changing the action configuration to use the GitHub Actions Bot also as author.

In addition, I am enabling commit signing and the sign-off in the commit message so that DCO can pass.

Submitter Checklist

• Includes tests if functionality changed/was added
• Includes docs if changes are user-facing
• Set a kind label on this PR
• Release notes block has been filled in, or marked NONE

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Release Notes

NONE

[HeavyWombat]

/lgtm
/approve

[openshift-ci]

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