Update gitlab integration to mention token expiration

[gilzow]

Where on docs.platform.sh should be changed?

https://docs.platform.sh/integrations/source/gitlab.html
https://docs.upsun.com/integrations/source/gitlab.html

What exactly should be updated?

Add a section for updating the gitlab token in an integration. Mention the gitlab blog post regarding gitlab token lifetimes, then instructions on how to update the token value from the CLI as well as console.

Additional context

No response