deprecate(cmc): mark historic-rate endpoints as superseded by Governance

[jasonz-dfinity]

Why

Governance now computes the 30-day ICP/XDR average and maturity modulation locally (previous PRs). CMC is no longer the authoritative source for either value, so its rate-querying infrastructure is deprecated rather than removed to give downstream callers time to migrate.

https://dfinity.atlassian.net/browse/NNS1-4319

What

The following CMC items are marked deprecated (no code deleted; kept for upgrade safety and backward compatibility):

• Query endpoints: get_average_icp_xdr_conversion_rate, neuron_maturity_modulation
• Internal helpers: update_recent_icp_xdr_rates, compute_average_icp_xdr_rate_at_time, compute_maturity_modulation, compute_capped_maturity_modulation
• State fields: average_icp_xdr_conversion_rate, recent_icp_xdr_rates, maturity_modulation_permyriad

Deprecated items are moved to the end of main.rs so active code appears first.

PR Chain

• ⬅️ Previous: refactor(governance): remove old CMC-based maturity modulation and XDR rate polling #9877

Testing

Existing CMC tests cover the deprecated code paths, which remain functional.

[Copilot]

Pull request overview

Deprecates legacy CMC “historic-rate” state fields and query endpoints that are now superseded by NNS Governance’s locally computed ICP/XDR averages and maturity modulation, while retaining functionality for upgrade safety and backwards compatibility.

Changes:

• Added deprecation documentation to persisted StateV2 fields that Governance no longer reads.
• Marked the historic-rate query endpoints as #[deprecated] and clarified they’re kept for backwards compatibility.
• Replaced helper-function doc comments with deprecation guidance pointing callers to Governance implementations.

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[Copilot]

The doc comment references UpdateIcpXdrRatesTask in NNS Governance, but that type doesn’t appear to exist. In Governance, the scheduled timer task is UpdateIcpXdrRateRelatedData (see rs/nns/governance/src/timer_tasks/mod.rs:15,44). Please update the reference so the deprecation guidance points to the correct replacement.

Suggested change

	/// Deprecated: Superseded by `UpdateIcpXdrRatesTask` in NNS Governance, which maintains
	/// Deprecated: Superseded by `UpdateIcpXdrRateRelatedData` in NNS Governance, which maintains

[Copilot]

This comment says Governance uses compute_average_icp_xdr_rate from the ic_nervous_system_clients library, but the only compute_average_icp_xdr_rate found is in Governance (rs/nns/governance/src/timer_tasks/update_icp_xdr_rate_related_data.rs:61). Please correct the reference (crate/module) so readers can find the actual implementation.

Suggested change

	/// Deprecated: NNS Governance uses `compute_average_icp_xdr_rate` from the shared
	/// `ic_nervous_system_clients` library instead. CMC retains this for its own deprecated
	/// `update_recent_icp_xdr_rates` path.
	/// Deprecated: NNS Governance now uses its own `compute_average_icp_xdr_rate`
	/// implementation in Governance's ICP/XDR rate update task. CMC retains this for its
	/// own deprecated `update_recent_icp_xdr_rates` path.

[Copilot]

The deprecation comment points to update_icp_xdr_rates.rs, but Governance’s implementation lives in rs/nns/governance/src/timer_tasks/update_icp_xdr_rate_related_data.rs (and tests in ..._tests.rs). Please update the filename/module reference to avoid a dead pointer.

Suggested change

	/// in `update_icp_xdr_rates.rs`. CMC retains this for its own deprecated state.
	/// in `update_icp_xdr_rate_related_data.rs`. CMC retains this for its own deprecated state.