CI: mergify config (#1818)

<!-- .github/pull_request_template.md -->

## Description
Use case: If you merge(d) a PR to `dev` and also it's good to have it in
`main` ASAP. For example, tests fix.
Just add `backport_main` label to the PR (even if it's already merged).
The same PR will be created but targeted to `main`.

It's called
[Backport](https://docs.mergify.com/workflow/actions/backport/). If the
backport has conflicts - it will have a `conflicts` label. Be careful
with that

## Type of Change
<!-- Please check the relevant option -->
- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Code refactoring
- [ ] Performance improvement
- [ ] Other (please specify):

## Screenshots/Videos (if applicable)
<!-- Add screenshots or videos to help explain your changes -->

## Pre-submission Checklist
<!-- Please check all boxes that apply before submitting your PR -->
- [ ] **I have tested my changes thoroughly before submitting this PR**
- [ ] **This PR contains minimal changes necessary to address the
issue/feature**
- [ ] My code follows the project's coding standards and style
guidelines
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have added necessary documentation (if applicable)
- [ ] All new and existing tests pass
- [ ] I have searched existing PRs to ensure this change hasn't been
submitted already
- [ ] I have linked any relevant issues in the description
- [ ] My commits have clear and descriptive messages

## DCO Affirmation
I affirm that all code in every commit of this pull request conforms to
the terms of the Topoteretes Developer Certificate of Origin.

.mergify.yml

@@ -0,0 +1,10 @@
+pull_request_rules:
+  - name: Backport to main when backport_main label is set
+    conditions:
+      - label=backport_main
+      - base=dev
+      - merged=true
+    actions:
+      backport:
+        branches:
+          - main