DataCollection phase 3: deprecate SendDefaultPii

Part of #5420. Phase 3 — deprecation (minor release).

Deprecate `SentryOptions.SendDefaultPii` (`src/Sentry/SentryOptions.cs:333`):

- Changelog + doc-comment deprecation with the exact migration snippet replicating `SendDefaultPii = false` via `DataCollection` (the JS changelog for 10.57.0 is the template: getsentry/sentry-javascript#21277).
- Semantics: `SendDefaultPii = true` ≈ new defaults; if both options are set, `SendDefaultPii` is ignored.
- **Decision needed**: `[Obsolete]` produces compiler warnings that break `TreatWarningsAsErrors` builds (JS's JSDoc `@deprecated` had no such cost). Options: (a) `[Obsolete]` in the minor with a clear message, (b) doc-only deprecation in the minor and `[Obsolete]` in the next major. Discuss before landing.

Start at src/Sentry/SentryOptions.cs:333 and review the JS changelog template referenced in getsentry/sentry-javascript#21277. Confirm the DataCollection behavior and decide with maintainers whether the minor release uses [Obsolete] or doc-only deprecation. Done means the changelog and doc comment include the migration snippet, and the agreed deprecation semantics are documented.

Written by the indexing model from the issue text.