Fix deprecation warning about Content-Type header in Rails 6.1

What does this MR do?

This MR replaces content_type with media_type since it is going to be deprecated in Rails 6.1

This is one instance of the warnings this MR gets rid of:

DEPRECATION WARNING: Rails 6.1 will return Content-Type header without modification. If you want just the MIME type, please use `#media_type` instead. (called from block (4 levels) in <main> at /Users/aakriti/Development/gdk/gitlab/ee/spec/requests/git_http_geo_spec.rb:57)

Related to #206536 (closed)

Screenshots (strongly suggested)

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