Remove tabindex where not needed

What does this MR do?

Fix for #219385 (closed)

This MR removed tabindex from some elements, as a consequence the outline is also removed from them.

Screenshots

As a rule of thumb, we should not add tabindex to elements that are not interactive. However, one of our concerns was that screen readers would not pick up our headings, so I made a small test using Apple's VoiceOver to confirm this still worked:

VoiceOver Demo (sound on please!)

In regards to our tabbed navigation, this is a demo of the flow, starting from the top of the dashboard:

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