fix(protocol-versions): allow equal activation timestamps

[PelleKrab]

Summary

Loosens ProtocolVersions timestamp ordering from strictly increasing to non-
decreasing for previous scheduled upgrades.

Motivation

This supports testnets/devnets that activate multiple upgrades at genesis with the
same timestamp.

Behavior

• 0 remains the unscheduled value and is skipped by ordering checks.
• Non-zero timestamps must be >= the closest previous scheduled timestamp.
• Timestamps before the previous scheduled upgrade still revert.
• The existing strict check against the next scheduled upgrade is unchanged.

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2