Use `application/texinfo` as sole MIME type for `.texi` and `.texinfo` files

[clin1234]

Per https://www.iana.org/assignments/media-types/application/texinfo, the file format has an officially-registered MIME type, thus deprecating the application/x-texinfo alias.

Linked PRs

• gh-140166: Use application/texinfo as sole MIME type for .texi and .texinfo files #140165