eCash(XEC) Network upgrade to latest major version (0.26.x)

In order to activate reliably at a predictable time, the network upgrade uses the "Median Time Past" mechanism. The upgrade activates when the median of the last 11 blocks reaches timestamp 1668513600 (12:00:00 UTC on Nov 15th, 2022). This means that the upgrade doesn't actually activate exactly at that time, but typically about one hour later, when 6 blocks with timestamps greater than the activation time have been produced.