Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Critical pod priorityClass addition
**What this PR does / why we need it**:
@bsalamat - Apologies for the delay. This PR is to ensure that all pods with priorityClassName `system-node-critical` and `system-cluster-critical` will be critical pods while preserving backwards compatibility.
**Special notes for your reviewer**:
- Moved some constants and other data structures to scheduler/api/types.go where other constants are present.
- An automatic assignment of critical priorities to pods based on critical pod annotation for backwards compatibility including some unit tests.
xref: https://github.com/kubernetes/kubernetes/issues/57471
**Release note**:
```release-note
Critical pods to use priorityClasses.
```
