Migrate JSON libraries to `encoding/json/v2`

[darccio]

Discussed in https://github.com/DataDog/dd-trace-go/discussions/4187

^{Originally posted by darccio November 28, 2025}

Package Name

No response

Package Version(s)

No response

Describe the feature you'd like

After working on #4176, we concluded that it'd be good to migrate from the following libraries to encoding/json/v2:

• github.com/json-iterator/go
• github.com/minio/simdjson-go

Is your feature request related to a problem?

Reduce our dependencies to the bare minimum.

Describe alternatives you've considered

No response

Additional context

No response

[eliottness]

After working on https://github.com/DataDog/dd-trace-go/pull/4176, we concluded that it'd be good to migrate from the following libraries to encoding/json/v2

Adding a bit more of context: We don't want to migrate from github.com/minio/simdjson-go because this is significantly faster than any other libraries but we need a fallback library because it does not support a lot of architectures and encoding/json is currently slower than github.com/json-iterator/go so we chose this one.

[darccio]

Additional context: in order to migrate, the new library must support partial parsing.