fix(fixture): updated api.github.com/markdown

BREAKING CHANGE: api.github.com/markdown has changed

```diff
[
   {
     headers: {
-      x-commonmarker-version: "0.20.0"
+      x-commonmarker-version: "0.21.0"
     }
   }
   {
     headers: {
-      x-commonmarker-version: "0.20.0"
+      x-commonmarker-version: "0.21.0"
     }
   }
 ]
```

scenarios/api.github.com/markdown/normalized-fixture.json

@@ -28,7 +28,7 @@
       "referrer-policy": "origin-when-cross-origin, strict-origin-when-cross-origin",
       "status": "200 OK",
       "strict-transport-security": "max-age=31536000; includeSubdomains; preload",
-      "x-commonmarker-version": "0.20.0",
+      "x-commonmarker-version": "0.21.0",
       "x-content-type-options": "nosniff",
       "x-frame-options": "deny",
       "x-github-request-id": "0000:00000:0000000:0000000:00000000",
@@ -63,7 +63,7 @@
       "referrer-policy": "origin-when-cross-origin, strict-origin-when-cross-origin",
       "status": "200 OK",
       "strict-transport-security": "max-age=31536000; includeSubdomains; preload",
-      "x-commonmarker-version": "0.20.0",
+      "x-commonmarker-version": "0.21.0",
       "x-content-type-options": "nosniff",
       "x-frame-options": "deny",
       "x-github-request-id": "0000:00000:0000000:0000000:00000000",