Switch custom unicode folding with golang.org/x/text/cases

[l0nax]

Currently, this code base has a custom Unicode folding implemented in util/unicode_case_folding.go.
But this feature has already been implemented with golang.org/x/text/cases.

I'm proposing switching to the golang/x package because, for one, the golang.org/x/text/cases implementation is much performant and smaller. We're using goldmark in our application, and it is the biggest contributor to the binary size (the map in the utils package).

[yuin]

goldmark's one of the most important policy is 'Depends only on standard libraries.' (as README says)