Update Doorkeeper to version 5.8.0

What does this MR do and why?

• This requires us to vendor doorkeeper-openid_connect until https://github.com/doorkeeper-gem/doorkeeper-openid_connect/pull/213 is merged
• The only difference between the vendored version and the gem version we were on before is the gemspec dependency. I also added one small note in the vendored gem README about why it was vendored
• We are updating Doorkeeper in order to use the dynamic scopes feature https://github.com/doorkeeper-gem/doorkeeper/pull/1739
• https://my.diffend.io/gems/doorkeeper/5.7.1/5.8.0

Changelog: changed

References

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MR acceptance checklist

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Screenshots or screen recordings

OIDC still works locally with these changes (this is an auth code from my GDK, not a publicly accessible GitLab instance):

How to set up and validate locally

To test that OIDC is still working with these changes:

1. Log into gitlab as an administrator
2. Create an oauth application with the openid scope at GDK_ROOT/admin/applications
3. Visit https://oidcdebugger.com/ and enter the following:
   1. Authorize_url: GDK_ROOT/oauth/authorize
   2. Redirect URI: https://oidcdebugger.com/debug
   3. Client id: client id from created oauth app
   4. Scope: openid
   5. Response type: select code
4. Select "send request" button to test OIDC flow