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

[sl0thentr0py]

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 (feat(pii): Soft-deprecate send_default_pii + backwards-compatible mapping #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.

[linear-code]

RUBY-198