Move "include external files in .gitlab-ci.yml" from Starter to Core

This is a community request to move the recently introduced feature #20868 (closed) from Premium to Core.

Problem to Solve

GitLab previously decided that "include" feature in gitlab-ci.yml is a feature "that [is] most relevant for large organizations" and therefore belongs to GitLab Premium. https://about.gitlab.com/stewardship/#what-features-are-ee-only

This feature however, in terms of reusing code/definitions, is important to all good implementations of CI. Furthermore, bringing this feature into broader availability will ensure the CI definitions are more compatible between CE/EE, promoting code reuse and shared templates without having to bifurcate the user base or provide CE and EE separate examples.

Solution

We will update this feature to be available for all CE users