envoy_reloadable_features_use_migration_in_quiche deprecation

[publish-envoy]

Your change commit abfa0c9 (use QUICHE built-in port and Server Preferred Address migration) introduced a runtime guarded feature. It has been 6 months since the new code has been exercised by default, so it's time to remove the old code path. This issue tracks source code cleanup so we don't forget.