T8458: enabled pr mirroring

[kumvijaya]

Proposed Commit Message

Enabled PR mirror to replicate pullrequests automatiocally from public to private repo

summary: no more than 70 characters

A description of what the change being made is and why it is being
made, if the summary line is insufficient.  The blank line above is
required. This should be wrapped at 72 characters, but otherwise has
no particular length requirements.

If you need to write multiple paragraphs, feel free.

LP: #NNNNNNN (replace with the appropriate bug reference or remove
this line entirely if there is no associated bug)

Additional Context

Test Steps

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[c-po]

identical workflow as already used in other repos like vyos-1x

[asklymenko]

Our common workflow for PR mirroring.