chore(mergify): add merge_bot_account to enable automatic merging (#35217)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

Mergify needs a designated bot account to automatically execute merge
operations for PRs that meet the merge queue conditions. Without this
configuration, automatic merging may not function properly.

### Description of changes
Configure AWS CDK Automation as the merge bot account to allow Mergify
to automatically execute merge operations for PRs that meet the queue
conditions.
Added `merge_bot_account: AWS CDK Automation` to the `.mergify.yml`
configuration file. This designates the AWS CDK Automation account as
the bot responsible for executing automatic merges when PRs meet the
defined queue conditions.

### Describe any new or updated permissions being added

NA


### Checklist
- [x] My code adheres to the [CONTRIBUTING
GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and
[DESIGN
GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license*

Co-authored-by: Jorge Diaz <[email protected]>

Author: alvazjor

.mergify.yml

@@ -1,4 +1,5 @@
 # See https://doc.mergify.io
+merge_bot_account: aws-cdk-automation
 merge_queue:
   # Hopefully this gets Mergify to stop creating PRs just to test merges (it both messes with statistics and our automatic validations)
   max_parallel_checks: 1