Motivation removal of V2 API in #12913

Context Etcd supports Discovery protocol https://etcd.io/docs/v3.4/dev-internal/discovery_protocol/ based on V2 API. In v3.6 we are removing V2 API so we need a replacement that will work in v3.6+ versions. In v3.6 discovery v2 should still be able to do on older vclusters

Goal:

• Add --discovery-version flag that accepts v2 (default) and v3 value. Using v3 will switch Etcd to use discovery protocol using V3 API
• Update documentation https://etcd.io/docs/v3.4/dev-internal/discovery_protocol/
• Mark for v2 discovery as deprecated

Deprecation plan:

• Make v3 default in v3.7
• Remove v2 option in v3.8