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>

assets/licenses.yaml

@@ -182,7 +182,7 @@ dependencies:
 
   - name: markdown
     kind: dart-package
-    version: "7.2.2"
+    version: "7.3.1"
     homepage: https://pub.dev/packages/markdown
     license: BSD-3-Clause
     purpose: >-