Upgrade doorkeeper-openid_connect to 1.8.10

* We were vendoring the gem before in order to fix version dependency
  issues with doorkeepeer (before it didn't support 5.8), see
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/173481
* Those version dependency issues were fixed in
  https://github.com/doorkeeper-gem/doorkeeper-openid_connect/pull/214
* Those change were released with v 1.8.10 of the gem