Fallback to GET validators from state on 400 POST response

[StefanBratanov]

PR Description

We are handling fallback to GET validators from state on status codes 404 and 405 in #7786, but LH seems to be returning 400, so handling that case as well, which should cover all scenarios.

│ teku java.util.concurrent.CompletionException: tech.pegasys.teku.validator.remote.FailoverRequestException: Remote request (get_validator_statuses) failed on all configured Beacon Node endpoints 
│ teku ...: java.lang.IllegalArgumentException: Invalid params response from Beacon Node API (url = ...)
│ ...:5051/eth/v1/beacon/states/head/validators, status = 400, message = BAD_REQUEST: Unsupported endpoint version: v1)

Also add flag --Xuse-post-validators-endpoint-enabled until all clients implement this endpoint.

Fixed Issue(s)

N/A

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[zilm13]

LGTM