refactor: deprecate V1 signin, verify email and invite APIs

Currently the following endpoints are not being used by the dashboard
`/user/signin`
`/user/verify_email`
`/user/invite`

New APIs are created for the above APIs which are
`/user/v2/signin`
`/user/v2/verify_email`
`/user/invite_multiple`

The old APIs are kept for backwards compatibility, as the movement is completed entirely from the FE side as well, old APIs can be removed.