Update mergify config

[leonschoorl]

It was reporting:

The configuration uses the deprecated update_method attribute of the queue action in one or more pull_request_rules. It must now be used under the queue_rules configuration.
A brownout is planned on August 26th, 2024.
This option will be removed on September 23rd, 2024.
For more information: https://docs.mergify.com/configuration/file-format/#queue-rules

[leonschoorl]

I'm unsure whether this is right, or it should be something like:

...

queue_rules:
  - name: backport patches to 1.2
    update_method: rebase
  - name: backport patches to 1.4
    update_method: rebase
  - name: backport patches to 1.6
    update_method: rebase
  - name: backport patches to 1.8
    update_method: rebase

Or if we even need this at all.
The volume of backports typically isn't very high, so merge queue feel like unnecessary complexity.

[martijnbastiaan]

IIRC it took me a couple of tries to get the config correct too, and there is no great way of testing bar making a separate repo. So I think we can just merge and try it out.

[leonschoorl]

Apparently the deadline was extended

A brownout is planned on September 16th, 2024.
This option will be removed on October 21st, 2024.

[martijnbastiaan]

Let's merge anyway?