Rename all templates to *.gitlab-ci.yml

To make use of the autocompletion and linting available via the official GitLab extension for VSCode (I'm sure similar things exist for other IDEs) we should rename all of the templates in this project to follow the *.gitlab-ci.yml naming convention as this is how these extensions and linters etc. will determine that a specific yaml file is in fact a gitlab-ci type file.

Obviously, this is a breaking change. So, we would need to bump the repo to a new v3.0.0 release.

Here is an example of the vscode autocomplete suggesting the predefined CI variables in a drop-down list #winning!