Disable definition lists in Markdown (#17648)

This PR disables definition list support in `pulldown_cmark`, as it is
has been causing a number of issues.

I opened an issue upstream with the panic we were seeing:
https://github.com/pulldown-cmark/pulldown-cmark/issues/957.

Release Notes:

- N/A

crates/markdown/src/parser.rs

@@ -5,10 +5,13 @@ use pulldown_cmark::{Alignment, HeadingLevel, LinkType, MetadataBlockKind, Optio
 use std::ops::Range;
 
 pub fn parse_markdown(text: &str) -> Vec<(Range<usize>, MarkdownEvent)> {
+    let mut options = Options::all();
+    options.remove(pulldown_cmark::Options::ENABLE_DEFINITION_LIST);
+
     let mut events = Vec::new();
     let mut within_link = false;
     let mut within_metadata = false;
-    for (pulldown_event, mut range) in Parser::new_ext(text, Options::all()).into_offset_iter() {
+    for (pulldown_event, mut range) in Parser::new_ext(text, options).into_offset_iter() {
         if within_metadata {
             if let pulldown_cmark::Event::End(pulldown_cmark::TagEnd::MetadataBlock { .. }) =
                 pulldown_event