feat(pii)!: Remove `send_default_pii` & adopt spec PII-on defaults (next major)

**Follow-up — NOT part of the current stack. Backlog / next major.**

Required by the initiative's Definition of Done.

## Scope (next major only)

* Remove the `send_default_pii` accessor and its backwards-compatible mapping (getsentry/sentry-ruby#3002).
* Flip `data_collection` defaults to the **spec's PII-on model** (collect user identity, HTTP bodies, cookies, query params by default, with per-category opt-out).
* Remove the soft-deprecation warning.
* Update docs + migration guide for the major bump.

*Breaking change. Do not implement during the current minor-release migration.*