Use new isodatetime copy methods

[MetRonnie]

Needs:

• [ ] https://github.com/metomi/isodatetime/issues/209 addressed + a corresponding isodatetime release

We currently have at least 1 case of mutating isodatetime data classes, however they are supposed to be immutable. We should switch over to the proposed .copy() methods that will allow changing of atttributes. See https://github.com/cylc/cylc-flow/pull/4554#discussion_r839766062