Add EC2 to AutoDevOps template

What does this MR do?

This MR modifies Auto-Build (Build.gitlab-ci.yml), asking the user to create their own build job (named build_artifact) so that AutoDevOps for EC2 deployment can be fully enabled.

Screenshots

Prerequisites: AUTO_DEVOPS_PLATFORM_TARGET is set to EC2.

Sample project with a 'build_artifact' job

.gitlab-ci.yml file of sample project: https://gitlab.com/ebaque/jekyll-demo/-/blob/deploy-to-ec2/.gitlab-ci.yml

build_artifact:
  stage: build
  image: ruby:2.3
  before_script:
    - bundle install
  script:
    - bundle exec jekyll build -d public
  artifacts:
    paths:
      - public

Pipeline: https://gitlab.com/ebaque/jekyll-demo/-/pipelines/210615125

Sample project without any defined 'build_artifact' job

Pipeline: https://gitlab.com/ebaque/jekyll-demo/-/pipelines/210619953 Without a build_artifact job, and after triggering a pipeline, the build stage fails with the following error message:

Failed job: https://gitlab.com/ebaque/jekyll-demo/-/pipelines/210615125

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #216008 (closed)