Add notice at the beginning of issue templates asking reporters to check for duplicates first

What does this MR do?

This adds a notice to ask reporters to check for duplicates before creating an issue.

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

People might not remove the notice. I thought of putting it in a <details></details> block but then readers would be tempted to open them to check it it's the notice or not, so it would be counter-productive. I think if people don't remove the notice, that's probably not a big deal...

Why was this MR needed?

Because we get a lot of duplicates.

Closes #31549 (closed)