DataCollection phase 5: remove SendDefaultPii (next major)

[jamescrosswell]

Part of #5420. Phase 5 — next major.

Remove SendDefaultPii and make the spec defaults unconditional:

• Remove SentryOptions.SendDefaultPii, BindableSentryOptions.SendDefaultPii, the Serilog sendDefaultPii sink parameter, and the legacy bridge in the resolver.
• Permissive spec defaults apply to everyone; migration guide entry for users who relied on SendDefaultPii = false (point at the restrictive DataCollection block).
• Simplify native propagation (map from resolved config only).
• JS analogue: v11 cleanup, getsentry/sentry-javascript#22056.

[linear-code]

NET-774