GitLab Integration does not work with actual Service accounts

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

1. Create a [GitLab service account](https://docs.gitlab.com/user/profile/service_accounts/) as suggested by the [documentation](https://docs.sentry.io/organization/integrations/source-code-mgmt/gitlab/#install)
2. Setup the GitLab application
3. Configure the Sentry integration
4. Wait for a place to put in the Personal access token for the service account...

### Expected Result

There is a way to provide the PAT for a GitLab (service) account, that will then own the integration.

### Actual Result

Configuration is completed by redirecting to the OAuth flow, with no option to put in an PAT instead. As actual gitlab service accounts don't support logging in through the UI or OAuth authentication, it seemingly is only possible to use a regular account setup as a service account (which costs a seat).

### Product Area

Settings - Integrations

### Link

_No response_

### DSN

https://4cef07b5d17ebdf4509ab48792b79ad8@o4507058143428608.ingest.de.sentry.io/4507063739416656

### Version

_No response_

No file or test is named. Start by tracing the Settings - Integrations GitLab setup from its OAuth redirect and identify where authentication credentials are selected or stored. Done means a GitLab service account can provide its personal access token and own the integration without interactive OAuth or a paid regular-user seat.

Written by the indexing model from the issue text.