ci: add mergify backport rule for 9.4 branch

[jmikell821]

This PR just adds 9.4 to mergify so that PRs labeled backport-9.4 can auto-create a backport PR.

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• run exhaustive tests : Run the exhaustive tests Buildkite pipeline.

[mergify]

This pull request does not have a backport label. Could you fix it @jmikell821? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[donoghuc]

Ah, interesting. I think this exposes a bug. I believe we attempted to automate this with

logstash/.github/workflows/version_bumps.yml

Lines 72 to 77 in 3c4fbaf

	- name: Create GitHub backport label (Mergify) (minor only)
	if: ${{ inputs.bump == 'minor' }}
	continue-on-error: true
	run: make -C .ci backport-label BACKPORT_LABEL=$BACKPORT_LABEL
	env:
	GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}

but it is in the wrong action. I'll chase that down separately.