[8.x](backport #4306) github-actions: copy gh labels to the mergify backports

[mergify]

What is the problem this PR solves?

Copy GitHub labels from the original PR to the mergify backports.

How does this PR solve the problem?

With this proposal, then #4228 will contain the labels from #4227

How to test this PR locally

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

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This is an automatic backport of pull request #4306 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[mergify]

This pull request has not been merged yet. Could you please review and merge it @v1v? 🙏

[michalpristas]

sorry merged before noticing review from @v1v is missing.
if victor does not approve or has some issues today by the end of the date i will revert and recreate