Remove quote wrapping limitation from KAS docs

What does this MR do and why?

This change set removes a documented limitation that projects ids in the agent configuration YAML mustn't be wrapped in quotes. However, this is not the case.

It was introduced in Quote marks in file path aren't allowed (!110898 - merged) but I was not able to understand why. The MR nor the referenced ticket mentions the "why" and the ticket has some other issues.

We've also had some users confused by this, see Gitlab Kubernetes agent failed to parse agent c... (gitlab-org/cluster-integration/gitlab-agent#521 - closed)

I think that we should await the users feedback in gitlab-org/cluster-integration/gitlab-agent#521 (closed) before we merge this. Just in case there really is some kind of weird issue that we are not aware of. However, in that case I suggest that we document the why.