Removing mouse up and mouse down from modal Backdrop component (Shopify#8090)

[Spin instance](https://polaris.polaris-f45t.emmanuel-etti.us.spin.dev/)

### WHY are these changes introduced?
- Closes Shopify#8144
- Modal component closes when a user selects within the modal (for
example, in an input field while selecting typed input) but overshoots
the modal boundaries and releases their mouse outside the modal (in the
backdrop).

### WHAT is this pull request doing?
- Removes the `onMouseUp` and `onMouseDown` handles from `Backdrop`
component. There is a test that covers this case of explicitly clicking
the backdrop to close it and we think that is sufficient behaviour.

### Before


https://user-images.githubusercontent.com/78582921/213541746-f3304f21-737f-411b-aea1-a8cc35264e57.mov

### After


https://user-images.githubusercontent.com/78582921/214117861-d8866a19-b45d-4d08-a94f-cd467199a3b5.mov

### 🎩 checklist

- [ ] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [x] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [x] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)

Co-authored-by: Jason Kurian <jason.kurian@shopify.com>
Co-authored-by: Kyle Durand <kyledurand@users.noreply.github.com>

Author: 3 people

.changeset/gentle-chefs-hear.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Replaced mouse up and down events on Backdrop with onClick to close Modal

polaris-react/src/components/Backdrop/Backdrop.tsx

@@ -29,7 +29,7 @@ export function Backdrop(props: BackdropProps) {
     }
   };
 
-  const handleMouseUp = () => {
+  const handleClick = () => {
     if (setClosing && onClick) {
       setClosing(false);
       onClick();
@@ -41,10 +41,9 @@ export function Backdrop(props: BackdropProps) {
       <ScrollLock />
       <div
         className={className}
-        onClick={onClick}
+        onClick={handleClick}
         onTouchStart={onTouchStart}
         onMouseDown={handleMouseDown}
-        onMouseUp={handleMouseUp}
       />
     </>
   );

polaris-react/src/components/Backdrop/tests/Backdrop.test.tsx

@@ -4,11 +4,13 @@ import {mountWithApp} from 'tests/utilities';
 import {Backdrop} from '..';
 
 describe('<Backdrop />', () => {
-  describe('onDismiss()', () => {
+  describe('onClick()', () => {
     it('is called when the backdrop is clicked', () => {
       const spy = jest.fn();
-      const backdrop = mountWithApp(<Backdrop onClick={spy} />);
-      backdrop.find('div', {onClick: spy})!.trigger('onClick');
+      const backdrop = mountWithApp(
+        <Backdrop onClick={spy} setClosing={() => {}} />,
+      );
+      backdrop.find('div')!.trigger('onClick');
       expect(spy).toHaveBeenCalled();
     });
   });