[TIKA-4724] Use IANA-registered text/markdown as primary type

[ec1oud]

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)

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