Update omniauth to 2.0

Summary

omniauth has a new major version 2.0 which might be a breaking changethough we need to remove unmaintained omniauth-azure-oauth2 to update

gems with compatible upstream release

• omniauth-github - update to 2.0
• omniauth-gitlab - update to 3.0
• omniauth-saml - update to 2.0
• omniauth-oauth (indirect dependency via omniauth-twitter) - update to 1.2

gems without compatible upstream release

• omniauth-azure-oauth2 - should be removed (replaced by omniauth-azure-activedirectory-v2)
• omniauth-cas3 - upstream not ready - https://github.com/dlindahl/omniauth-cas/issues/62
• omniauth_crowd - upstream not ready - https://github.com/robdimarco/omniauth_crowd/issues/37
• omniauth-salesforce - upstream not ready - https://github.com/hunter-io/omniauth-salesforce/commit/9151768c4d098ea41c61e098926057e103e7f630
• gitlab_omniauth-ldap - needs update - https://gitlab.com/gitlab-org/omniauth-ldap
• omniauth-multipassword (indirect dependency of omniauth-kerberos) - upstream not ready - https://github.com/jgraichen/omniauth-multipassword/issues/3

Improvements

Full improvements in upstream release page linked above

Risks

typemaintenance