Return Collection from #variables

What does this MR do?

• Changes Ci::Build#variables and Ci::Build::Context::Build/Ci::Build::Context::Global to return a Variables::Collection object instead of an array of hashes;
• Removes #scoped_variables_hash since it now it is possible to just call #scoped_variables.to_hash;
• Adds Ci::BuildRunnerPresenter#to_runner_variables and exposes that as :variables in JobRequest::Response;
• Extends ExpandVariables#transform_variables to handle receiving a Variables::Collection;

Changes previous implementation to directly return a Collection from the existing #variables property and discards the recently introduced #variables_collection, as well as #scoped_variables_hash, which isn't really needed.

Note: There's an alternative MR that could replace both !54509 (merged) and this MR, in case we prefer that approach.

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

Part of gitlab-runner#26345 (closed)