Remove "sha" argument from GraphQL ciConfigVariables

What does this MR do and why?

In !116032 (merged), we deprecated the sha argument from ProjectType.ci_config_variables and updated the FE to use the new ref argument. This was done as a first step to gracefully deprecate sha so that there aren't issues with the FE if it's a version behind the latest BE release.

Now that the MR thereof has been pushed to production, and the ProjectType.ci_config_variables is in alpha, we can proceed immediately with removing the sha argument altogether.

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 use sha (ciConfigVariables(sha: $ref)), you get an error output.

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 #404493 (closed)