Backend: Deprecate "sha" parameter from CI config-variables

As a follow-up from !109639 (merged), we need to do these;

• Backend: Add the ref parameter to the Types::ProjectType.ci_config_variables field.
• Backend: Deprecate the sha parameter of the Types::ProjectType.ci_config_variables field.
• Frontend: Use the ref parameter instead of sha.
• Backend: Remove the Projects::PipelinesController#config_variables endpoint: Almost no usage in the last 7 days: https://log.gprd.gitlab.net/goto/e1bbdde0-9bcd-11ed-9f43-e3784d7fe3ca

The sha argument is to be removed in #404493 (closed). (This follows the suggested practice of graceful deprecation.)

Implementation

	Description	MR
backend frontend	Deprecate ProjectType.ci_config_variables sha arg; add ref arg - This solves the first 3 items listed in this issue.	!116032 (merged)
backend	Remove the Projects::PipelinesController#config_variables endpoint	!116044 (merged)