Do not check for valid url for .gitlab-ci.yml

Closes #22970

See merge request !8451

Until this commit, GitLab would check if the URL in your .gitlab-ci.yml
would be valid and adressable. However, this approach is rather limited.

To begin, the URL couldn't be http://example:$PORT/my-project, as the
PORT value would be interpolated after checking if the URL was valid,
which its not. Also this gem will limit the amount of protocols allowed.

This commit doesn't check, at all, if the URL is valid. This is now the
responsablilty of the user. In my opion this is right, as the user
ultimately benefits from a correct URL, but also, its impossible to
cover all cases.

So that this would be more consistent with the other variables,
which all of them are string based.

Closes #25554