Add CI testing against Ruby 3.4 and 4.0

[larouxn]

Proposing we add CI testing against both Ruby 3.4 and 4.0 given ruby-head is now Ruby 4.1. Note, I wasn't sure what to do with the wintests section so I updated it the same but can revert if desired.

[larouxn]

I believe we need to first do the following before we'll see passing CI here.

• Drop support for ok_json and json_pure as multi_json has done #66
• Drop support for EOL rubies i.e. those older than 3.3.x #67

Unfortunately, even with both of those changes on a branch, CI is still failing so it's likely an actual issue perhaps in dependencies or code, given Ruby versions and multi_json changes are handled.

https://github.com/larouxn/json-ld/actions/runs/27867108935/job/82472935416

[larouxn]

Update, I have gotten CI passing for standard Ruby but doing so required four code level changes. As I'm not familiar with this gem nor RDF, just wanted to migrate from multi_json to json and do a bit of maintenance, I would appreciate some guidance as to what do to do. I can open PRs with the fixes or I'd be fine with a maintainer picking them off my branch (main commit: 14a3525) or doing some other kind of fix. In the end we should just aim to get CI into a passing state.