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: >-

pubspec.lock

@@ -302,10 +302,10 @@ packages:
     dependency: "direct main"
     description:
       name: markdown
-      sha256: ef2a1298144e3f985cc736b22e0ccdaf188b5b3970648f2d9dc13efd1d9df051
+      sha256: ee85086ad7698b42522c6ad42fe195f1b9898e4d974a1af4576c1a3a176cada9
       url: "https://pub.dev"
     source: hosted
-    version: "7.2.2"
+    version: "7.3.1"
   matcher:
     dependency: transitive
     description:
@@ -625,4 +625,4 @@ packages:
     version: "3.1.3"
 sdks:
   dart: ">=3.12.0 <4.0.0"
-  flutter: ">=3.32.0"
+  flutter: ">=3.35.0"

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