Activate Startup.CSS

What does this MR do?

This deactivates all Startup.css checks and activates it fully in production. All bigger problems have been adressed so far and we can move to full activation with this MR. All known problems at the moment are only if you hit non-cached and very specific pages or situations. (see #235497 (closed)).

As follow up we will polish those topics, improve for different themes and also the size of the injected CSS.

At the same time we will be constantly reducing the main application.css size which should lead then to the performance improvements.

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