[TIKA-4724] Use IANA-registered text/markdown as primary type (#2806)

text/markdown has been the IANA-registered media type for Markdown
since RFC 7763 (March 2016). freedesktop's shared-mime-info has used
it as the primary type for years, with text/x-markdown as an alias.
Consumers that fall back to Tika's bundled database when no system
shared-mime-info is installed (e.g. Qt on macOS and Windows) cannot
currently resolve text/markdown and silently drop the type from
filter lists.

Promote text/markdown to the primary type and keep Tika's historical
text/x-web-markdown together with freedesktop's text/x-markdown as
aliases, so existing lookups continue to resolve. Update the comment
to match freedesktop's wording.

Generated-by: Claude Opus 4.7 (claude-opus-4-7)
(cherry picked from commit c297247)

Author: ec1oud

tika-core/src/main/resources/org/apache/tika/mime/tika-mimetypes.xml

@@ -8533,8 +8533,10 @@
     <sub-class-of type="text/plain"/>
   </mime-type>
 
-  <mime-type type="text/x-web-markdown">
-    <_comment>Markdown source code</_comment>
+  <mime-type type="text/markdown">
+    <_comment>Markdown document</_comment>
+    <alias type="text/x-web-markdown"/>
+    <alias type="text/x-markdown"/>
     <glob pattern="*.md"/>
     <glob pattern="*.mdtext"/>
     <glob pattern="*.mkd"/>