feat(governance): enable Mission 70 voting rewards

[jasonz-dfinity]

rs/nns/governance/src/lib.rs: replace Cell::new(cfg!(feature = "test")) with Cell::new(true).

Effects that turn on:

• Max neuron dissolve delay capped at 2 years, with existing neurons clamped (feat(nns): Reduce maximum dissolve delay to 2 years and clamp all neurons #9688)
• Voting rewards pool scaled by 0.6329 (feat(governance): Use Mission 70 voting rewards purse adjustment. #9698, later re-calculated)
• Quadratic dissolve delay bonus, replacing linear (feat(governance): Quadratic dissolve delay bonus for Mission 70. #9719)
• Reduced minimum dissolve delay required to vote; vote/propose thresholds decoupled (feat(nns): Reduce neuron_minimum_dissolve_delay_to_vote_seconds to 2 weeks #9689, feat(governance): decouple minimum dissolve delay to vote from propose #9880)
• 8 year gang 10% voting power bonus (feat(nns): expose eight_year_gang_bonus_base_e8s in neuron info and full neuron #9687, feat(nns): apply 8 Year Gang 10% bonus to voting power calculation #9896)
• Voting power snapshots cleared on activation (feat(governance): clear voting power snapshots on Mission 70 activation #9731)

[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
you have considered the following (for Governance-owned code):

1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

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[Copilot]

Pull request overview

This PR enables the Mission 70 voting rewards behavior in NNS governance by switching the runtime flag default to “on”, and updates the unreleased changelog to reflect the activation.

Changes:

• Enable ENABLE_MISSION_70_VOTING_REWARDS by default in rs/nns/governance/src/lib.rs.
• Add a release-note entry describing the Mission 70 activation in unreleased_changelog.md.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
rs/nns/governance/unreleased_changelog.md	Adds an “Enabled Mission 70 voting rewards” entry describing activated behaviors.
rs/nns/governance/src/lib.rs	Flips the Mission 70 voting rewards feature flag default to true (production-enabled).

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[bjoernek]

I think this PR superseded by
#9923