Users importing from Bitbucket Cloud, Bitbucket Server, or GitHub
often complained about getting failed imports due to 422 errors.
This change ensures that project names are imported with names that
are guaranteed to pass the regular expression validation.

Part of #50021

This should use stub_omniauth_setting, not do it manually.

Also use Gitlab::Auth::OAuth::Provider.config_for to access OmniAuth config

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.

Signed-off-by: Rémy Coutable <remy@rymai.me>