Remove legacy PDPVerifier AFTER_EPOCH bootstrap flow

[rjan90]

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