deps: bump markdown 7.2.2 -> 7.3.1, unblocked by the Dart 3.9 floor (T-353)

Now that the SDK floor is Dart 3.9 (markdown 7.3.1 requires ^3.9.0), take
the bump. 7.3.0/7.3.1 are bug-fixes + perf with no breaking AST/Node
changes (we walk the parsed tree in our own renderer); only API-level
deprecations (LinkContext, BlockParser.standardBlockSyntaxes) we don't
use. licenses.yaml updated in the same commit; markdown goldens green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -54,10 +54,7 @@ dependencies:
 
   jovial_svg: 1.1.30
 
-  # Held at 7.2.2: 7.3.1 requires Dart ^3.9.0, which would raise our
-  # environment floor below; only minor fixes behind it, no advisory. Bump
-  # alongside a deliberate SDK-floor decision (T-353).
-  markdown: 7.2.2
+  markdown: 7.3.1
 
   flutter_widget_from_html_core: 0.17.2