Convert stylesheets/pages/admin.scss into page specific bundle

What does this MR do?

• Extract admin/metrics_and_profiling CSS to separate page bundle.
• Extract admin/jobs_index CSS to separate page bundle.
• Clean up admin.css and remove it completely

Screenshots (strongly suggested)

Section	Before	After
Admin > Dashboard
Admin > Jobs > Index		No visual changes
Admin > Settings > Metrics and Profiling		No visual changes

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 #239360 (closed)