configuration: Rename validation_upgrade_{frequency -> cooldown} (#4635)

This just renames a member of `HostConfiguration` from
validation_upgrade_frequency to -//-_cooldown.

As was already pointed out in #4460 the existing name is a misnomer, the
member actually represents a minimum time period between upgrades, which
is neatly expressed by a word cooldown.

I've been planning this rename already for some time and the term is
already used in paras module:

https://github.com/paritytech/polkadot/blob/1394b70d493a3b4bea7a9ae14af094081bc84456/runtime/parachains/src/paras.rs#L1568-L1574