.mergify/config.yml: Remove rebase_fallback attribute (deprecated)
PR builds and CI are currently broken due to a mergify brownout
today because edk2 uses the `rebase_fallback` attribute of the
`queue` action.
Message from Mergify/Summary:
```
The configuration uses the deprecated rebase_fallback attribute
of the queue action.
A brownout is planned on February 13th, 2023.
This option will be removed on March 13th, 2023.
For more information: https://docs.mergify.com/actions/queue/
```
Therefore, this change removes the attribute per the guidance in
the following changelog message to retain existing behavior.
https://changelog.mergify.com/changelog/rebasefallback-is-deprecated
```
The option rebase_fallback is now deprecated and should not be
used anymore.
Mergify will always report errors in the future if a rebase merge
is impossible.
```
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Rebecca Cran <rebecca@bsdio.com>