Draft: Accept .gitlab-ci.yaml as the default CI Config

What does this MR do and why?

Today, we only accept .gitlab-ci.yml as the default CI Config. However, making .gitlab-ci.yaml default is also asked from users. With this change, we also accept .gitlab-ci.yaml as a default source for CI config if .gitlab-ci.yml does not exist in the project.

This change is behind a feature flag ci_gitlab_ci_yaml_as_default (#428592 (closed)).

Related to #26169

Screenshots or screen recordings

An example project;

Before enabling ci_gitlab_ci_yaml_as_default

After enabling ci_gitlab_ci_yaml_as_default

MR acceptance checklist

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