backport: Add 8.4 branch

[v1v]

Merge as soon as 8.4 branch was created. Auto-merge is not yet supported, see Mergifyio/mergify#2821

[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-v./d./d./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]

This PR was created by running:

RELEASE_TYPE=patch \
  CURRENT_RELEASE=8.4.2 \
  PROJECT_OWNER=elastic \
  PROJECT=elastic-agent \
  make -C release_scripts prepare-next-release create-prs-next-release

@amannocci , I'm not sure if this PR is correct for patch releases since the existing backport labels are major.minor.0 based instead. Additionally the automation for the minor release 8.4.0 was already done in #784.

Would you mind double checking if this is the expected behaviour? Otherwise, would you mind revisiting the scripts to reflect the required changes? 🙏

[v1v]

unexpected value!?

[elasticmachine]

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• Start Time: 2022-09-16T08:49:44.860+0000

• Duration: 17 min 27 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4935
Skipped	17
Total	4952

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	97.333% (73/75)	👍
Files	68.085% (160/235)	👍
Classes	68.478% (315/460)	👍
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Conditionals	100.0% (0/0)	💚

[cmacknz]

The same block already exists below at line 223, likely because this shouldn't run for patch releases, only major/minor releases that create a new branch.

[fearful-symmetry]

Yah, assuming this can be closed. Tell me and I'll re-open it if I'm wrong, hah.

[amannocci]

This PR was created by running:

RELEASE_TYPE=patch \
  CURRENT_RELEASE=8.4.2 \
  PROJECT_OWNER=elastic \
  PROJECT=elastic-agent \
  make -C release_scripts prepare-next-release create-prs-next-release

@amannocci , I'm not sure if this PR is correct for patch releases since the existing backport labels are major.minor.0 based instead. Additionally the automation for the minor release 8.4.0 was already done in #784.

Would you mind double checking if this is the expected behaviour? Otherwise, would you mind revisiting the scripts to reflect the required changes? 🙏

I will check the behavior of the automation. AFAIK, we have two things wrong here. The PR should be raised only for minor/major releases and the merge within the mergify file isn't correct.