Make planning epoch config change in BABE easier (#5776)

* Make planning epoch config change in BABE easier

* Bump node runtime version

* Update frame/babe/src/lib.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* BabeEpochConfiguration -> NextConfigDescriptor

* Add tests for babe config changes

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>
Co-authored-by: André Silva <andre.beat@gmail.com>