cshtml: use HTML5 instead of XHTML :fried_egg:

[jamacku]

HTML5 is more user-friendly and modern

Discovered in:

• Provide html output with detected defects 🌐  redhat-plumbers-in-action/differential-shellcheck#400

off-topic: Could please create a csdiff issue to port cshtml to html5 instead of xhtml. Thank you! - redhat-plumbers-in-action/differential-shellcheck#400 (comment)