Combine constants into CI_JOB_USER

What does this MR do?

There is no advantage to two constants: The registry login also uses find_for_git_client (via JwtController), and the registry login also uses the job token as the passsword. So just use CI_JOB_USERNAME for user and job token for password in all cases.

See also !37899 (merged)

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.