Consider supporting Twitter OAuth2 integration

Contrary to our documentation (https://docs.gitlab.com/ee/integration/twitter.html), GitLab only supports Twitter OAuth 1.0a, not 2.0.

Twitter recently added OAuth 2.0 support (https://developer.twitter.com/en/docs/authentication/oauth-2-0):

Currently we use https://github.com/arunagw/omniauth-twitter, but this gem appears to have gone unmaintained as it does not support OmniAuth v2.0.

https://github.com/unasuke/omniauth-twitter2 may be an option for OAuth 2.0 support. Existing identities would likely not work here since the client ID and secret are different.