Apply better hierarchy to markdown headers and issue/mr titles

What does this MR do?

Applies better hierarchy to markdown headers and issue/mr titles

Why was this MR needed?

Titles of issues/mr's were smaller than h1, leading to strange hierarchy when reading descriptions

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• 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 #21395 (closed)