update markdown version

Changed `markdown.md` VERSION from "28.1 2025-02-02"
to "28.2 2025-04-28".

Updated sitemap.

Tested the above via `make www gen_status gen_sitemap timestamp` on macOS.

sitemap.xml

@@ -15490,11 +15490,11 @@
 </url>
 <url>
     <loc>https://www.ioccc.org/markdown.html</loc>
-    <lastmod>2025-04-28T20:00:17+00:00</lastmod>
+    <lastmod>2025-04-28T20:19:21+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/markdown.md</loc>
-    <lastmod>2025-04-28T19:59:57+00:00</lastmod>
+    <lastmod>2025-04-28T20:19:11+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/news.html</loc>
@@ -15598,7 +15598,7 @@
 </url>
 <url>
     <loc>https://www.ioccc.org/status.html</loc>
-    <lastmod>2025-04-28T20:09:44+00:00</lastmod>
+    <lastmod>2025-04-28T20:19:27+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/status.json</loc>