ci: when updating readme, use original author

[kd7lxl]

What this PR does:
When Github Actions updates the README on a Renovate PR, Renovate detects the new author and no longer updates the PR. Example: #612 (comment)

Edited/Blocked Notification
Renovate will not automatically rebase this PR, because it does not recognize the last commit author and assumes somebody else may have edited the PR.

You can manually request rebase by checking the rebase/retry box above.

⚠️ Warning: custom changes will be lost.

This PR reuses the original author metadata so that Renovate ignores the commit and will happily rebase the PR. It also adds a co-authored-by line so that it's easy to distinguish as a Github Actions commit.

Which issue(s) this PR fixes:
Fixes #

Checklist

• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX], [DEPENDENCY]