Rollup merge of #111152 - lukas-code:markdown-parsers-are-hard, r=GuillaumeGomez

update `pulldown-cmark` to `0.9.3`

This PR updates `pulldown-cmark` to version `0.9.3`, which does two main things:
* Pulls in pulldown-cmark/pulldown-cmark#643 to fix rust-lang/rust#111117
* Allows parsing strikethrough with single tildes, e.g. `~foo~` -> ~foo~. This matches the [GFM spec](https://github.github.com/gfm/#strikethrough-extension-).

Full changelog: pulldown-cmark/pulldown-cmark#646

Author: compiler-errors