Update to xxhash 0.8.3 and ammend Ractor tests

[dysonreturns]

Hi.

This change aims to bring forward some performance updates from xxhash since 0.8.1 particularly on arm, by updating to the latest release 0.8.3.

Upgrade path:
I noticed which files libxxhash.c and libxxhash.h correspond to in the official repo and updated them like this:

curl -L https://raw.githubusercontent.com/Cyan4973/xxHash/refs/tags/v0.8.3/xxhash.c -o ./libxxhash.c
curl -L https://raw.githubusercontent.com/Cyan4973/xxHash/refs/tags/v0.8.3/xxhash.h -o ./libxxhash.h

Feel free to verify they match exactly.

Tests: The Ractor test failed for Ruby 4, which now has slightly different Ractor syntax, so there's now a conditional for Ruby 3 and Ruby 4 in the spec.

Manual tests per Ruby: I built and spot-checked Rubies 3.2.x, 3.3.x and 4.0.x. They all appear fine.

I wasn't able to compile for Ruby 2.x on my MacOS, but that might just be me. I'm not sure whether 2.x is supported, even. Maybe consider marking a min Ruby version in the gemspec if that's the case?

3.2.x is end-of-life shortly, so 3.3 is a good target.

spec.required_ruby_version = '>= 3.3.0'

[nashby]

@dysonreturns thanks! I'll add 4.0 to CI. Ruby 2 is not officially supported though so it's fine. I'll consider adding required version config.