Deprecate ProjectType.ci_config_variables `sha` arg; add `ref` arg

What does this MR do and why?

This MR solves the first 3 items listed in: Backend: Deprecate "sha" parameter from CI conf... (#389065 - closed)

This MR adds the argument ref to the Types::ProjectType.ci_config_variables field and deprecates sha, as it is actually supposed to be ref.

sha will be removed in a future issue to minimize the possibility of incompatibility if the FE version is out of sync with the BE.

How to set up and validate locally

1. Set up a project whose .gitlab-ci.yml file has the following contents:

variables:
  CONFIG_VAR_1: 'Value for CONFIG_VAR_1'
  CONFIG_VAR_2: 'Value for CONFIG_VAR_2'

job:
  script: echo

2. Go to http://gdk.test:3000/-/graphql-explorer and run the following query (update fullPath as necessary).

query getProjectConfigVariables($fullPath: ID!, $ref: String!) {
  project(fullPath: $fullPath) {
    id
    ciConfigVariables(ref: $ref) {
      key
      value
    }
    __typename
  }
}

Variables:

{"fullPath": "group-a/project-1", "ref": "master"}

3. Observe that the result shows both config variables:

4. Also observe that if you update the query to ciConfigVariables(sha: $ref), you get the same result.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #389065 (closed)