feat: automate unified release by issue creation

[amannocci]

What is the change being made?

• Create an issue template as entrypoint to trigger the unified release.

Why is the change being made?

• We will implemented an upstream GitHub workflow that will create issue in each repository on Unified Release Day.
• Issue tracking will be easy since we will only need to wait for all issues to be closed.

[mergify]

This pull request does not have a backport label. Could you fix it @amannocci? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[v1v]

Before moving forward, can we send the design document to the rest of the team?

[v1v]

I'm not sure if this is the right sentence since it's not about a release today, but it could be a Feature Freeze

[v1v]

If the parent issue is passed, do you think the GH PR that was created could also contain a reference to it and also a reference to the GitHub issue in this project that was created?

### Issues

Part of https://github.com/elastic/dev/issues/xxx

Closes https://github.com/elastic/apm-server/issues/xxx

[amannocci]

There are pros/cons in both cases.
In my opinion, we could keep the parent relationship between those issues and let the PR reference the project issue to avoid confusion in the upstream issue.