Retire old `webserver` RPC client and use new everywhere

[pcrespov]

A follow-up from PR #8431 will be required to clean up redundant code

• remove non-v1
• add new client in payments and directorv2
• remove default webserver namespace
• redesign api-server mock mechanism for rpc etc
• redesign webserver's app_environment fixture. Right now is very error prone
  - Use dicts and filter via docker-service section.