As https://openid.net/specs/openid-connect-core-1_0.html#toc mentions,
OpenID Connect Discovery endpoints should support the use of Cross
Origin Resource Sharing so that single-page JavaScript applications can
work with GitLab. These endpoints, which are provided by the
`doorkeeper-openid_connect` gem, now have CORS headers:

- `/oauth/userinfo`
- `/oauth/discovery/keys`
- `/.well-known/openid-configuration`
- `/.well-known/webfinger`

In addition, this commit adds CORS headers to:

- `/oauth/revoke`

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/49756, we added
added CORS headers for `/oauth/token` to support OAuth Proof Key for
Code Exchange (PKCE) flow. Adding `/aouth/revoke` is an extension of
that work.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/300077

Changelog: added