Fix safari scrolling issue (#7395)

### WHY are these changes introduced?

Scrolling and interacting with pages using index table may be slugging
on (newish versions?) of safari.

### Screenies

||Before|After|
|-|-|-|
|Timeline|<img width="356" alt="Screen Shot 2022-10-13 at 11 51 05 AM"
src="https://user-images.githubusercontent.com/24610840/195645166-7835c563-f756-46eb-a16e-6784b998defb.png">|<img
width="427" alt="Screen Shot 2022-10-13 at 11 57 29 AM"
src="https://user-images.githubusercontent.com/24610840/195646407-cff19381-90ea-4ac8-a5d8-499e861ea247.png">|
|Layers|<img width="693" alt="Screen Shot 2022-10-13 at 10 42 45 AM"
src="https://user-images.githubusercontent.com/24610840/195645603-0fa69e9f-f2d2-465b-8bfc-165b926f636c.png">|<img
width="322" alt="Screen Shot 2022-10-13 at 10 49 27 AM"
src="https://user-images.githubusercontent.com/24610840/195645994-4c81a595-5db1-4ed7-acc6-87a1ff967b38.png">|
|Layer count|<img width="149" alt="Screen Shot 2022-10-13 at 10 43 46
AM"
src="https://user-images.githubusercontent.com/24610840/195645673-85f481b3-31c7-4c0b-92db-612c35d99cbe.png">|<img
width="324" alt="Screen Shot 2022-10-13 at 10 48 54 AM"
src="https://user-images.githubusercontent.com/24610840/195645856-8494841f-f4fb-4b36-a094-67cb41ef04bf.png">|


### WHAT is this pull request doing?

I'm removing the `filter` property that's causing a new layer to be
create for each row. Painting each layer is taking a considerable amount
of time. But why not use the GPU? Sure, we could. But taking advantage
of the GPU would cause a significant bump in memory usage. I tested out
`will-change: filter` and noticed a resting memory usage of ~1gb
occasionally going higher (or lower).

<img width="336" alt="Screen Shot 2022-10-13 at 10 44 26 AM"
src="https://user-images.githubusercontent.com/24610840/195647680-cf3b9594-86be-4ea0-9cae-fc58e8199abf.png">

#### Why are we running into the issue?

`box-shadow` seems to have performance issue with scrolling. I haven't
located a recent thread but found many older posts on slack overflow of
similar issues and [webkit bug
reports](https://bugs.webkit.org/show_bug.cgi?id=22102).

But aren't we using drop-shadow not box shadow? Yes! However, upon
looking at the
[spec](https://drafts.fxtf.org/filter-effects/#funcdef-filter-drop-shadow)
it seems the values are interpreted as box-shadow with an additional
length property.

### Notes

There's still a few drop-shadows in the index table component. Since
we're using collapsing borders for the table they're needed.

### How to 🎩

Use the snapshot in web!

### 🎩 checklist

- [ ] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [ ] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

Author: AndrewMusgrave

.changeset/twenty-turtles-decide.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Fixed scrolling performance for IndexTable on safari

polaris-react/src/components/IndexTable/IndexTable.scss

@@ -120,7 +120,7 @@ $loading-panel-height: 53px;
 .TableRow {
   background-color: var(--p-surface);
   cursor: pointer;
-  filter: drop-shadow(0 -1px 0 var(--p-divider));
+  border-top: var(--p-border-width-1) solid var(--p-divider);
 
   &.TableRow-unclickable {
     cursor: auto;