Changes

feat(pipeline) tidb ghpr_mysql_test job migrate to GCP (#4441)

## Summary

Migrate `pingcap/tidb/ghpr_mysql_test` to GCP runtime and mark the job
as migrated in prow config.

## Changes

1. Update pipeline structure in
`pipelines/pingcap/tidb/latest/ghpr_mysql_test.groovy`
- switch to `agent none` + stage-level k8s agents
- use `yaml pod_label.withCiLabels(POD_TEMPLATE_FILE, REFS)` instead of
`yamlFile`
- merge `Checkout` + `Prepare` into `Checkout & Prepare`
- keep existing latest job behavior (cache keys / ws cache / post
behavior) while applying GCP migration pattern

replay build:
`https://prow.tidb.net/jenkins/job/pingcap/job/tidb/job/ghpr_mysql_test/2`
  - result: `SUCCESS`
 (commit: 4ce3056)