Changes

ci(tidb): align pull_unit_test_next_gen pod resources (#4549)

## Summary
- align `pull_unit_test_next_gen` pod resource requests/limits with the
master presubmit unit test baseline
- keep the next-gen-specific `bazel-repository-cache` mount and `300Gi`
workspace PVC unchanged

## Testing
- not run (pod yaml change only)
 (commit: cd6ab4c)