Reject non-ASCII BINARY strings during JSON generation

[ydah]

Remove the BINARY-to-UTF-8 compatibility fallback and its outdated warning, completing the deprecation planned for 3.0 in #643.

require "json"

JSON.generate("日本".b)
# Before: warns about json 3.0 and returns "\"日本\""
# After: JSON::GeneratorError

JSON.generate("ascii".b)
# Still returns "\"ascii\""

Use the existing encoding error handling in C and Java, matching TruffleRuby. Cover binary strings as values and keys, while preserving ASCII-only input.

But, maybe now isn't the good time? WDYT?