Add Helm-2to3.gitlab-ci.yml to Auto DevOps

What does this MR do?

Adds the previously external Helm-2to3.gitlab-ci.yml template to GitLab and includes it in Auto-DevOps.gitlab-ci.yml.

Users without a .gitlab-ci.yml also need to be able to migrate. For some users, adding a .gitlab-ci.yml may not be an option.

• The jobs are only included when a very specific CI/CD variable is present.
• The jobs are safe to run multiple times.

Screenshots (strongly suggested)

Pipelines using the following .gitlab-ci.yml (simulating this MR's changes):

include:
  - template: Auto-DevOps.gitlab-ci.yml
  - file: lib/gitlab/ci/templates/Jobs/Helm-2to3.gitlab-ci.yml
    project: gitlab-org/gitlab
    ref: add-helm-2to3-ci-template

Plain pipeline (link)	Pipeline with MIGRATE_HELM_2TO3 set to true (link)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.