fix: non-reverting upgrade announcement

[wjmelements]

Toward FilOzone/filecoin-services#505
Reviewer @rjan90
Duplicates FilOzone/filecoin-services#547

Changes

• deprecate current announcement method
• new announce upgrade plan method
• share upgrade announcement logic between new and old method
• update tools, docs

[rjan90]

I reviewed this and opened #289 as a stacked follow-up targeting this branch. It updates the rollout checklist to use the relative-delay flow after bootstrap, validates and records the observed on-chain afterEpoch, and retains the currently required v3.4.0 legacy bootstrap path. Removal of that temporary compatibility path is tracked in #288.

@wjmelements, could you review and merge #289 into fix/no-revert-announcement before merging this PR?

I am approving #285 at its current head, conditional on #289 landing first. If this repository dismisses stale approvals when the stacked commit lands, a fresh approval will be required on the combined head.

[rjan90]

The Solidity change and tests look good. Approving at the current head, with #289 to land into this branch before merge so the rollout checklist uses the relative-delay flow and retains the required v3.4.0 bootstrap path.