feat: Deprecate the Modal component

[brandonlenz]

⚠️ Per #787 the modal component is to be relocated to a Truss-specific repo: The modal component will always be available in the git history, but after merging this PR, that will be the only place to find it.

Summary

The Modal Component is not a part of uswds and is therefore being removed from the library.

As this is a breaking change, it is targeting the V2 release of react-uswds

Related Issues or PRs

closes #787

How To Test

1. Checkout this branch

git fetch
git checkout bl-deprecate-modal-breaking-787

2. Verify all references to Modal are removed.
3. Example App runs (Modal and redux have been removed from the example app)

[trussworks-infra-zz]

	Warnings
⚠️	This PR does not include changes to storybook, even though it affects component code.
⚠️	This PR does not include changes to tests, even though it affects source code.

Generated by 🚫 dangerJS against 5e87f28

[lpsinger]

Where will Modal live in the future? I just started using react-uswds and I am using Modal.

[kalvinwang]

@lpsinger USWDS actually added a modal back to their library a year ago, so ReactUSWDS now contains a new version of the modal, as described here: #1622

[lpsinger]

Oh, I see! I'm using react-uswds 2.6.0, so I guess that I am already using the new Modal component. Thanks!