release: support 8.x releases

[v1v]

Motivation/summary

8.x is the base branch for running the releases from. main should be updated with the changelogs when a new minor and the mergify snippet.

For such, we are gonna use the backport merge policy with Mergify and create the PR to main with the backport-8.minor label, so once it's merged it will be backported automatically to the new minor branch.

This might be a bit chicken:egg problem, since the PR in main contains the changes in Mergify, but I expert Mergify to be clever enough to apply the new changes and the create the PR automatically....

Otherwise, we might need to create two PRs:

• one with the mergify changes
• another one with the changelog

So the first one needs to be merged first and the second one after.

Test

Given this PR, let's run the minor-release target using my forked repository for the hypothetical release of 8.18.0.

$ gh pr checkout 14392
$ export GITHUB_TOKEN=$(gh auth token)
$ make minor-release RELEASE_VERSION=8.18.0 PROJECT_OWNER=v1v BASE_BRANCH=8.x

And it created three PRs:

• 8.18: mergify v1v/apm-server#58
• 8.18: update docs, versions and changelogs v1v/apm-server#59
• 8.18: update docs v1v/apm-server#60

NOTE: I had to sync-up 8.x with the changes of this PR.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
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.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[v1v]

Even though the GH label creation happens somewhere else, let's force the backport to be created just in case. This will be later on used for the docs PR to be backported from main to 8.x and the new minor branch.