Changed all font-weight values to 400 and 600

What does this MR do?

Changes all font-weights respectively to either 400 or 600

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We have mixed usage throughout our application.. resulting in mixed weights. Let's simplify and make things consistent. This is one of the steps to have a proper typography system.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34138 (closed)