DataCollection phase 3: deprecate SendDefaultPii

[jamescrosswell]

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: ref(core): Deprecate sendDefaultPii in favor or dataCollection 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.

[linear-code]

NET-772