Remove legacy PDPVerifier AFTER_EPOCH bootstrap flow

## Context

PR #285 adds `announceUpgradePlan(address,uint96)`, but the currently deployed PDPVerifier v3.4.0 implementation does not expose that method. The first upgrade containing #285 must therefore retain a temporary `announcePlannedUpgrade` / `AFTER_EPOCH` bootstrap path.

The normal flow after that bootstrap should use `UPGRADE_DELAY_EPOCHS`, read the observed on-chain `nextUpgrade.afterEpoch`, and use that observed value for subsequent readiness checks.

## Remove when

- [ ] The Calibration proxy exposes `announceUpgradePlan(address,uint96)`.
- [ ] The Mainnet proxy exposes `announceUpgradePlan(address,uint96)`.
- [ ] Rollback to a pre-`announceUpgradePlan` implementation is retired, or its legacy procedure is documented separately.

## Cleanup

- [ ] Remove the temporary bootstrap sections from `.github/ISSUE_TEMPLATE/pdpverifier-upgrade.md`.
- [ ] Remove or separately archive `AFTER_EPOCH` support in `tools/announce-planned-upgrade.sh`.
- [ ] Remove the legacy compatibility text from `tools/README.md`.
- [ ] Make `UPGRADE_DELAY_EPOCHS` the only normal announcement path.
- [ ] Update associated tests and release documentation.

Related: #285

Start by checking whether the Calibration and Mainnet proxies expose announceUpgradePlan(address,uint96), then read the bootstrap sections in .github/ISSUE_TEMPLATE/pdpverifier-upgrade.md and the compatibility guidance in tools/README.md. Review tools/announce-planned-upgrade.sh, associated tests, and release documentation. Done means the temporary AFTER_EPOCH path and legacy text are removed or archived, with UPGRADE_DELAY_EPOCHS as the only normal announcement path.

Written by the indexing model from the issue text.