Switch to upstream omniauth_openid_connect gem

What does this MR do and why?

We forked the original gem a while ago into to fix a number of issues and add a number of features (https://gitlab.com/gitlab-org/ruby/gems/gitlab-omniauth-openid-connect/-/merge_requests?scope=all&state=merged).

Since then we've upstreamed all the changes into the omniauth_openid_connect (https://github.com/omniauth/omniauth_openid_connect) repository. In addition, the upstream project has added PKCE support and has other contributors.

This commit locks the openid_connect gem to v1.3.0 since upgrading past that version pulls in an updated net-smtp, which cannot be used with Ruby 2.7 due to https://bugs.ruby-lang.org/issues/17761. See https://docs.gitlab.com/ee/development/emails.html#rationale for more details.

Relates to gitlab-org/ruby/gems/gitlab-omniauth-openid-connect#5 (closed)

How to set up and validate locally

1. Check out gitlab-development-kit!2942 (merged) for GDK support of OIDC.
2. Register an OAuth2 application (e.g. in Google): https://docs.gitlab.com/ee/administration/auth/oidc.html. FYI, Google only allows real top-level domains or private IPs/hosts such as localhost.
3. Enter in the OAuth2 client ID and secret in client_options.identifier and client_options.secret.
4. Log into your account, enter -/profile/account.
5. Click on Connect OpenID Connect.
6. Authenticate with Google.
7. Sign out and log back in with OpenID Connect.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.