chore(ci): enable mergify branch protection (aws#35221)

### Reason for this change

<!--What is the bug or use case behind this change?-->

Mergify seems to be updated with latest branch protection rules, so they
can be enabled back to unblock mergify

### 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*

Author: ozelalisen

.mergify.yml

@@ -1,5 +1,4 @@
 # 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
@@ -23,7 +22,6 @@ queue_rules:
     commit_message_template: |-
       {{ title }} (#{{ number }})
       {{ body }}
-    branch_protection_injection_mode: none
 
   - name: priority-squash
     update_method: merge
@@ -44,7 +42,6 @@ queue_rules:
     commit_message_template: |-
       {{ title }} (#{{ number }})
       {{ body }}
-    branch_protection_injection_mode: none
 
   - name: default-squash
     update_method: merge
@@ -64,7 +61,6 @@ queue_rules:
     commit_message_template: |-
       {{ title }} (#{{ number }})
       {{ body }}
-    branch_protection_injection_mode: none
 
 pull_request_rules:
   - name: label core