Update for React 19

[AnnMarieW]

This is the start of the update to support React 19 in DMC

DO NOT merge until PR plotly/dash#3880 is available in Dash:

To do:

• Update webpack to handle external jsx runtime
• Build with the new webpack and run the dmc-docs app with React 19 using this branch and dash branch with PR 3880 (Result: work without any issues/errors)
• Update tests to use environment variable for react, rather than specifying in the app.
• Before release, be sure to build with the dash release containing PR 3880
• Release as a DMC v2 so React 19 can be run with DMC V2 (Mantine V8)
• Work on DMC v3 (Mantine v9) which requires React 19 in a separate PR.