Update codebase with use of new "Ultimate" plan along with the deprecated "Gold" plan

What does this MR do?

Relates #321364 (closed)

This MR replaces 'gold' occurrences with 'ultimate' in the specs/ files. Some files in app/ were also updated when it needed to as a result.

In some tests, 'ultimate' was added instead of replacing 'gold' (like here for example) because the nature of the plan (ie. the fact that it's called Gold vs Ultimate) was at the root of the test.

So I tried to draw line between replacing Gold vs adding Ultimate, I did my best to try to respect it.

As this MR is already pretty big, I will create another MR for Premium/Silver.

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

Related to #321364 (closed)