Convert backtick (`) admonition fences to tildes (~)

[ErikSchierboom]

In line with Exercism's spec, we're ensuring that all admonition fences are demarcated with four tildes (~~~~) across all repositories. We will be following up with an org-wide script that can be used to keep this consistent. Problem Specifications has already been updated.

We'll automatically merge this a week from now, but feel free to merge beforehand!

• Spec: https://exercism.org/docs/building/markdown/markdown#h-special-blocks-sometimes-called-admonitions
• Meta issue: https://github.com/exercism/exercism/issues/6705

[safwansamsudeen]

Incidentally, merging this can close #222.