Proposal: Retire `Charset` from the implicit scope in `EntityEncoder`

[danicheg]

This addresses #6561. The whole point is that Charset isn't typeclass, so it shouldn't appear in implicit scope. Though this breaks binary compatibility, this was aimed to don't fail compilation for users who use default EntityEncoder with UTF-8. Also, this wasn't discussed at all, so don't hesitate to point out that I'm wrong.