Mark the COLLECT search API as [Experimental] (NRS003); SE.Redis 3.0.25

[mgravell]

REDUCE COLLECT is still gated behind the server's search-enable-unstable-features flag, so its wire format can move under us. It shipped as stable public API in v1.7.0 by oversight; this re-flags it as experimental for 1.7.1.

Follows the established NRS ritual (cf. bed9558 for NRS002):

• Experiments.SearchCollect const, reusing id NRS003. That id briefly existed on marc/8_10 for server 8.10 but was removed before any release - confirmed absent from every v* tag - so it is free to reuse under the 'ids that reached users are never reused' rule, and gets no retired-id breadcrumb.
• [Experimental] on CollectReducer and Reducers.Collect().
• [NRS003] prefix on all 10 COLLECT entries in PublicAPI.Shipped.txt. The analyzer requires the prefix on every member of an experimental type, not just the type.
• docs/exp/NRS003.md, and NRS003 added to Directory.Build.props NoWarn.

Also bumps StackExchange.Redis 3.0.17 -> 3.0.25.

---

Note

Low Risk
No runtime logic changes to COLLECT—only experimental API metadata, docs, and a patch-level Redis client dependency bump; consumers may see new NRS003 compile warnings.

Overview
Reclassifies the REDUCE COLLECT aggregation API from stable public surface to experimental under diagnostic NRS003, because Redis still gates COLLECT behind search-enable-unstable-features.

CollectReducer and Reducers.Collect() are annotated with [Experimental], Experiments.SearchCollect is registered, all COLLECT members in PublicAPI.Shipped.txt are prefixed with [NRS003], and docs/exp/NRS003.md documents server requirements and suppression options. The repo adds NRS003 to global NoWarn in Directory.Build.props so the library build stays clean.

StackExchange.Redis is bumped from 3.0.17 to 3.0.25 in central package management.

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