[Icon] Update 'Mobile' icon (#11633)

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### WHY are these changes introduced?

Updates the design of the mobile icon to bring more clarity when paired
with `TabletIcon`

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  Context about the problem that’s being addressed.
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### WHAT is this pull request doing?

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### How to 🎩

🖥 [Local development
instructions](https://github.com/Shopify/polaris/blob/main/README.md#install-dependencies-and-build-workspaces)
🗒 [General tophatting
guidelines](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md)
📄 [Changelog
guidelines](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#changelog)

### 🎩 checklist

- [ ] Tested a
[snapshot](https://github.com/Shopify/polaris/blob/main/documentation/Releasing.md#-snapshot-releases)
- [ ] 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: heyjoethomas

.changeset/rare-cobras-destroy.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris-icons': patch
+---
+
+Update mobile icon design

polaris-icons/icons/MobileIcon.svg

@@ -7,4 +7,4 @@ authors:
   - Adam Whitcroft
   - Joe Thomas
 date_added: 2019-03-15
-date_modified: 2023-06-23
+date_modified: 2024-02-21