Using XML Date format instead of Internet Date format for Solid-PREP Notifications.

Swapped the `object` and `target`/`origin` properties in case of `Add`/`Delete` notifications respectively.

+ Notifications are identified by UUIDs.
+ Turtle notification template has the same fields as JSON-LD.
+ `notify:state` transmits Event-ID (which uniquely identifies a resource's state) and not E-Tag (which identify representations of the resource).
+ Parent reuses the generated Event-ID.

+ Event-ID is generated in PREP notification middleware itself.
+ No longer setting `Event-ID` response header upon resource mutation, thus reverting the middlewares for unsafe methods to their original state, before PREP was introduced.

Change the CI node-version to use caret range to ensure that tests are run on Node.js v20.

Values for `as:object`, `as:target` and `as:origin` predicates in the Turtle PREP-Notification template are IRI's, so they must be wrapped in angle brackets.

Co-authored-by: Sarven Capadisli <[email protected]>