Remove v20-dependent dead code in overlay/herder

[marta-lokhova]

There are several v20 checks in overlay/herder/tx queue. Now that the network is way past v20, all that code can be cleaned up. For example, we don't need to wait to externalize to initialize soroban tx queue (it doesn't need to be a pointer anymore). We should search for protocolVersionStartsFrom(protocolVersion, SOROBAN_PROTOCOL_VERSION) and remove dead code where appropriate.