Expose group runners toggle in Projects API

What does this MR do and why?

Exposes project.group_runners_enabled in the Project API endpoint, so it can be managed via the API and API-based tooling just like shared_runners_enabled.

Closes Add ability to "Disable group runners" at the p... (#367982)

🛠 with ❤ at Siemens

Screenshots or screen recordings

How to set up and validate locally

Using GDK:

# in ./gitlab/:

git remote add community https://gitlab.com/gitlab-community/gitlab.git
git fetch --all
git checkout community/feat/group-runners-enabled-api

gdk start

# obtain personal access token
 export GITLAB_TOKEN=glpat-your-token
curl -H "PRIVATE-TOKEN: $GITLAB_TOKEN" http://localhost:3000/api/v4/projects/gitlab-org%2Fgitlab-test | jq .group_runners_enabled
# true
curl -X PUT -H "PRIVATE-TOKEN: $GITLAB_TOKEN" http://localhost:3000/api/v4/projects/gitlab-org%2Fgitlab-test?group_runners_enabled=false | jq .group_runners_enabled
# false

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.