ci: autoqueue PRs and use ok-to-test label to queue

With these settings, Mergify should enable its internal
`allow_inplace_checks` option. This prevents creating special Merge PRs
for running CI jobs (hopefully).

Once this is merged, someone needs to add the `ok-to-test` label for
starting the CI jobs. There should not be any need for running
`@mergifyio queue` anymore.

Doc: https://docs.mergify.com/merge-queue/rules/#configuration-options
Signed-off-by: Niels de Vos <ndevos@ibm.com>

Author: nixpanic