Support Ruby 4.0 by upgrading magnus to 0.8

[mlarraz]

Summary

Ruby 4.0 removed the typed_flag field from RTypedData (see ruby/ruby@ce51ef30), which causes compilation failures with magnus 0.7.x:

error[E0609]: no field `typed_flag` on type `&rb_sys::RTypedData`
   --> magnus-0.7.1/src/r_typed_data.rs:202:58

This PR upgrades magnus from 0.7 to 0.8, which includes the fix (see matsadler/magnus#147).

Changes

• Bump magnus from 0.7 to 0.8 in both ext/parquet/Cargo.toml and ext/parquet-ruby-adapter/Cargo.toml
• Enable old-api feature to maintain compatibility with existing code that uses associated functions like RArray::new(), RHash::new(), Symbol::new() (deprecated in 0.8, not removed)
• Refactor StringCache to use Rust-side deduplication (HashSet<Arc<str>>) instead of Ruby VM interning via FString, which was removed in magnus 0.8 as it could not safely guarantee interned strings wouldn't be garbage collected
• Bump rb-sys from 0.9.116 to 0.9.124 for Ruby 4.0 bindings support

Testing

Verified the Rust extension compiles successfully against Ruby 4.0.1 on arm64-darwin.

Test plan

• cargo build --release compiles cleanly on Ruby 4.0.1
• Existing test suite passes on Ruby 3.x (backwards compatible — magnus 0.8 supports Ruby 2.6+)
• Existing test suite passes on Ruby 4.0