refactor: deprecate two-step postgres_changes API for OnPostgresChange

[Tr00d]

What

Deprecates the two-step postgres_changes registration API in favor of
OnPostgresChange (added in an earlier PR), which registers the option and
binds the handler in one call.

• [Obsolete("Favor OnPostgresChange instead.")] on RealtimeChannel.Register(PostgresChangesOptions)
  and AddPostgresChangeHandler, plus their IRealtimeChannel declarations.
• Client.Channel(database, schema, table, …) registers through an internal
  seam (RegisterPostgresChangesOptions) instead of the now-obsolete public
  Register, so the library itself stays CS0618-free.
• In-repo callers (tests + Examples/) moved onto OnPostgresChange to keep
  the zero-new-warnings bar.
• MIGRATION_v8.md records the deprecation and its replacement.

Why

OnPostgresChange supersedes the Register(...) + AddPostgresChangeHandler(...)
pairing (and the bare AddPostgresChangeHandler on an auto-* channel). Steering
callers there with a compile-time [Obsolete] is the deprecation half of that
migration, deferred out of the additive PR so the two land cleanly.

// before
channel.Register(new PostgresChangesOptions("public", "todos", ListenType.Inserts, filter));
channel.AddPostgresChangeHandler(ListenType.Inserts, handler);
// after
channel.OnPostgresChange(handler, ListenType.Inserts,
    new PostgresChangesFilter { Table = "todos", Filter = filter });

Not a removal

The methods still work — [Obsolete] is a compile-time warning only. Removal is
planned for the next major (v8), tracked in MIGRATION_v8.md.

Scope

Deliberately limited to the postgres_changes handler API. The draft #54 also
obsoleted PostgresChangesOptions.Parameters and the multi-param Channel(...)
overload; both are tangled (the Parameters ctor path, the channel-naming work)
and are left as separate follow-ups.

Relationship to #54

Fourth PR derived from the stale draft #54. #54 added the [Obsolete] markers but
left every in-repo caller emitting CS0618; this migrates them so the change is
actually warning-clean under our gate.

Testing

• Migrated postgres_changes tests exercise the same delivery behavior through
  OnPostgresChange; full realtime suite green.
• Clean build, zero new warnings; no known-vulnerable deps introduced.