BUG: text-encoding-identifier-case fixes <meta charset="UTF-8" /> to an invalid value

[taep96]

The text-encoding-identifier-case rule fixes <meta charset="UTF-8" /> to an invalid value ("utf8"). As per MDN:

charset

This attribute declares the document's character encoding. If the attribute is present, its value must be an ASCII case-insensitive match for the string "utf-8", because UTF-8 is the only valid encoding for HTML5 documents. elements which declare a character encoding must be located entirely within the first 1024 bytes of the document.

The only valid values would be UTF-8 and utf-8, not utf8

[fregante]

Duplicate of #1926