Bump markdown to 87e4c689342d5bbaa84ba2999abf0bff503979be

Changes:
```
> git log --format="%C(auto) %h %s" 32a9450..87e4c68
 https://dart.googlesource.com/markdown.git/+/87e4c68 Bump to 6.0.1 (#462)
 https://dart.googlesource.com/markdown.git/+/37305cf Add deprecated accessors for indicatorForUncheckedCheckBox and indicatorForCheckedCheckBox (#459)
 https://dart.googlesource.com/markdown.git/+/2e5e347 lint cleanup (#456)
 https://dart.googlesource.com/markdown.git/+/a0b5a05 example: small tweak to example – add Dart code (#455)
 https://dart.googlesource.com/markdown.git/+/de32a69 visitText to use textContent getter so subclassing Text class works correctly. (#431)
 https://dart.googlesource.com/markdown.git/+/8247955 Rewrite checkbox(task list) extension (#450)
 https://dart.googlesource.com/markdown.git/+/a602760 Fix crash with mixed checkbox lists (#449)

```

Diff: https://dart.googlesource.com/markdown.git/+/32a94505ded3863b82310b86344a003a19c00341~..87e4c689342d5bbaa84ba2999abf0bff503979be/
Change-Id: I92cf05fe6057e1b3eda8da09fc4eac4f67dd47ab
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/260441
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Auto-Submit: Devon Carew <devoncarew@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

DEPS

@@ -125,7 +125,7 @@ vars = {
   "linter_rev": "f2c55484e8ebda0aec8c2fea637b3bd5b17258ca", # 1.28.0
   "lints_rev": "8294e5648ab49474541527e2911e72e4c5aefe55",
   "logging_rev": "f5d64426af1b48966a4b33e97927c9fc456960b6",
-  "markdown_rev": "32a94505ded3863b82310b86344a003a19c00341", # b/236358256
+  "markdown_rev": "87e4c689342d5bbaa84ba2999abf0bff503979be",
   "markupsafe_rev": "8f45f5cfa0009d2a70589bcda0349b8cb2b72783",
   "matcher_rev": "6a9b83bbd73e50df2058b3e8e4aa301df49569c6",
   "mime_rev": "0a75a41445eb642674a0a271eecde78cb025ee60",