Update the default version of bitcoind to 0.21.1.
Deprecate support for version 0.18.1 and 0.19.1.

Nothing impacting changed between 0.20.1 and 0.21.1.
The only change relevant to lightning would be the following:

A new fee_rate parameter/option denominated in satoshis per vbyte (sat/vB) is introduced to the sendtoaddress, sendmany, fundrawtransaction and walletcreatefundedpsbt RPCs as well as to the experimental new send RPC.

But we will be able to leverage it only once we deprecate versions of bitcoind older than 0.21.1.

There's also a change that may be useful in tests (in particular for the TxPublisher):

The generateblock RPC allows testers using regtest mode to generate blocks that consist of a custom set of transactions. (#17693)