Add and use DeploymentName(). Add missing includes in blockchain.cpp. Rename `BIP9SoftForkDescPushBack` -> `SoftForkDescPushBack`
This is a partial backport of [[https://github.com/bitcoin/bitcoin/pull/19438 | core#19438]]
https://github.com/bitcoin/bitcoin/pull/19438/commits/92f48f360da5f425428b761219301f509826bec4
https://github.com/bitcoin/bitcoin/pull/19438/commits/8ee3e0bed5bf2cd3c7a68ca6ba6c65f7b9a72cca
Notes:
- D5218 already replaced the name argument with a DeploymentPos in BIP9SoftForkDescPushBack
- we don't report buried deployment in getblockchaininfo, so this backport only maintains the current code we have for active BIP9 activations. A `DeploymentName` function is added for buried deployments, but currently unused.
Depends on D12222