Backend: Remove deprecated predefined CI_BUILD_* variables

Proposal

The CI_BUILD_* variables were deprecated in GitLab 9.0, which is so long ago they've actually been completely removed from the docs a lot time ago, as being considered long out of the service. Unfortunately, we never actually removed the code, so they are still being added to every job. As these still technically exist, there could conceivably be people using them still, so we should make a full deprecation notice, and then remove them in 16.0. We will also provide the failed pipeline the notification in case any of them are using the removed variables.

Breaking Change

This will be a breaking change, but the only thing users need to do is update their variables to the current equivalents:

8.x name	9.0+ name
CI_BUILD_BEFORE_SHA	CI_COMMIT_BEFORE_SHA
CI_BUILD_ID	CI_JOB_ID
CI_BUILD_MANUAL	CI_JOB_MANUAL
CI_BUILD_NAME	CI_JOB_NAME
CI_BUILD_REF	CI_COMMIT_SHA
CI_BUILD_REF_NAME	CI_COMMIT_REF_NAME
CI_BUILD_REF_SLUG	CI_COMMIT_REF_SLUG
CI_BUILD_REPO	CI_REPOSITORY_URL
CI_BUILD_STAGE	CI_JOB_STAGE
CI_BUILD_TAG	CI_COMMIT_TAG
CI_BUILD_TOKEN	CI_JOB_TOKEN
CI_BUILD_TRIGGERED	CI_PIPELINE_TRIGGERED

NOTE: CI_BUILDS_DIR should not change, it is used to configure the runner's build directory when executing.

In addition in case the user are using one of the removed variables, we are going to add a detailed error message which explains why this pipeline is failing.

Removal will be done behind a Feature Flag, which will be enabled in %16.0.

Implementation

	Description	MR / Issue
documentation	Add deprecation notice for CI_BUILD_* variables	!80813 (merged)
backend	Prepare removal of legacy predefined CI_BUILD_* variables	!116606 (merged)
backend	[Feature flag] Roll out ci_remove_legacy_predefined_variables	Issue #404533 (closed)
backend	Set FF ci_remove_legacy_predefined_variables to default ON	!120050 (merged)
documentation	Add variable removal notice	!120073 (merged)