Use DNT: 1 as an experiment opt-out mechanism

What does this MR do?

GitLab runs pseudoanonymous experiments on our users, tracking data on various things. Currently, there is no way to opt out of being involved in these experiments. I raised this in #119107 and got agreement for an opt-out mechanism using DNT: 1 to be implemented. This MR implements that decision.

Screenshots

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

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

Closes #119107