chore: remove FDv2 pre-release warnings for GA promotion

[keelerm84]

Summary

• FDv2 / data-saving mode is going GA per the SDK Release Standards spec. GA-stage features must not carry experimental / pre-release warning prose in their public API surface.
• Removes the pre-release warning prose ("not stable, and not subject to any backwards compatibility guarantees", "not suitable for production usage", "in early access. ...join the EAP") from YARD comments on FDv2-related types in lib/ldclient-rb/interfaces/data_system.rb, lib/ldclient-rb/impl/data_system/protocolv2.rb, lib/ldclient-rb/integrations/file_data.rb, lib/ldclient-rb/integrations/test_data_v2.rb, and lib/ldclient-rb/config.rb. Descriptive YARD comments are preserved.

Tracking: SDK-2349

Test plan

• ruby -c clean on each changed file
• CI green

---

Note

Low Risk
Low risk: changes are limited to documentation/YARD comments and do not alter runtime behavior or protocol logic.

Overview
Promotes FDv2/data-saving mode to GA by removing experimental/early-access warning prose from the public API documentation.

Updates YARD comments across Config::DataSystemConfig, FDv2 protocol types (ProtocolV2), FDv2 integrations (FileData#data_source_v2, TestDataV2), and Interfaces::DataSystem constants/types, while preserving the remaining descriptive docs.

^{Reviewed by Cursor Bugbot for commit e25e9d7. Bugbot is set up for automated code reviews on this repo. Configure here.}