Enforce prevention of multiple instances of a locality in the same priority

*Title*: Enforce prevention of multiple instances of a locality in the same priority

*Description*:
PR #20574 refined the `LocalityLbEndpoints` structure to be defined as:
```
// A group of endpoints belonging to a Locality.
// One can have multiple LocalityLbEndpoints for a locality, but only if
// they have different priorities.
```
but Envoy allows to have multiple `LocalityLbEndpoints` for a locality if the different groups have different weights or different priorities.

Envoy should be more restrictive, and confirm to the revised comment.

cc @markdroth