Update CI/CD templates to stop using hardcoded `master`

Problem

devopscreate is going to change the default branch name from master to main in 14.0. This subsequently breaks the CI/CD templates that has a hardcoded only: master.

Example of potential impacts:

User creates a new project and picks a CI/CD template from UI, however, pipelines won't be triggered. In this case, the user has to fix the .gitlab-ci.yml by themself.
User creates a new project and include a template, however, the included jobs won't be triggered. In this case, the user has to override the only or rules in the template.

We should investigate which the templates should be refactored in 14.0.

Proposal

As part of the move away from master to main default branches (see linked Epic), we need to update our CI/CD templates to match.

This can be done immediately for:

Templates that are for copy/paste examples.
latest templates: https://docs.gitlab.com/ee/development/cicd/templates.html#latest-version

This should be delayed to 14.0 for breaking changes:

Templates that are used with include
stable templates: https://docs.gitlab.com/ee/development/cicd/templates.html#stable-version

Templates that use `only` (but not except):

Before:

only:
  - master

After:

rules:
  - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

Templates that use `except` (and possibly `only`)

These need to be handled differently. If you do a simple replacement like above, but with !=, the job will also run in MR pipelines. When #300146 is complete, we can probably change these to simple rules. For now, as an MVC, perhaps we should go with:

Before:

except:
  - master

only
  - master

After:

except:
  variables:
    - $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

only:
  variables:
    - $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

Template list

From a quick search by Shinya, we need to review/update all these templates, as a start:

14.0 breaking changes

/gitlab/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml -> Code comment links only, update with 14.0
/gitlab/lib/gitlab/ci/templates/Docker.gitlab-ci.yml -> Likely breaking change for include style template. Do in 14.0?
/gitlab/lib/gitlab/ci/templates/Managed-Cluster-Applications.gitlab-ci.yml -> Likely breaking change for include style template. Do in 14.0?
/gitlab/lib/gitlab/ci/templates/Terraform.latest.gitlab-ci.yml -> Just a link to https://gitlab.com/gitlab-org/gitlab/blob/master/, update in 14.0

Another Milestone (if time needed)

Reference

Edited May 20, 2021 by Marcel Amirault