Drop serde_json arbitrary_precision from workspace

[clockwork-labs-bot]

Closes #4989

We were not using arbitrary_precision in the workspace: I only found the workspace dependency declaration itself plus a normal serde_json::Number::from(3u8) call in crates/cli/src/spacetime_config.rs, which does not require the feature.

Summary

• remove serde_json/arbitrary_precision from the workspace dependency declaration
• keep raw_value, which is the only serde_json feature the workspace is clearly using here
• avoid forcing a non-additive serde_json feature onto downstream consumers via feature unification

Validation

• cargo tree -e features -p serde_json
• cargo check -p spacetimedb-sdk -p spacetimedb-lib -p spacetimedb-sats -p spacetimedb-schema

[gefjon]

I'm marking this as an API break because it does, technically, change user-visible behavior of using our library as a dependency. We should still do it (assuming CI shows that we are indeed not depending on this behavior), but it bears a release note.

[bfops]

It passes CI so I'm approving

I was looking at the wrong PR.. waiting for CI to actually pass.