🩹(react) harmonize date picker components output

By introducing this utility function, we ensure that any output value
is converted back to an ISO 8601 date and time string in UTC timezone.
Overall, it harmonize and factorize the way we output
values from date picker components.

.changeset/swift-forks-exercise.md

@@ -0,0 +1,5 @@
+---
+"@openfun/cunningham-react": patch
+---
+
+Fix datepicker component's output timezone management