[Typography foundations] Add responsive styling for heading2xl-4xl variants on <Text> (#7070)

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### WHY are these changes introduced?

This PR is a revision of #6919 without the un-needed commits

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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#local-development)
🗒 [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)

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<details>
<summary>Copy-paste this code in <code>playground/Playground.tsx</code>:</summary>

```jsx
import React from 'react';

import {Page, Text} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      <Text as="h1" variant="heading4xl">
        Heading4xl
      </Text>
      <Text as="h1" variant="heading3xl">
        Heading3xl
      </Text>
      <Text as="h1" variant="heading2xl">
        Heading2xl
      </Text>
    </Page>
  );
}
```

</details>

### 🎩 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: chazdean

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

@@ -93,18 +93,33 @@
 }
 
 .heading2xl {
-  font-size: var(--p-font-size-500);
-  line-height: var(--p-font-line-height-5);
+  font-size: var(--p-font-size-300);
+  line-height: var(--p-font-line-height-3);
+
+  @media #{$p-breakpoints-md-up} {
+    font-size: var(--p-font-size-500);
+    line-height: var(--p-font-line-height-5);
+  }
 }
 
 .heading3xl {
-  font-size: var(--p-font-size-600);
-  line-height: var(--p-font-line-height-6);
+  font-size: var(--p-font-size-400);
+  line-height: var(--p-font-line-height-4);
+
+  @media #{$p-breakpoints-md-up} {
+    font-size: var(--p-font-size-600);
+    line-height: var(--p-font-line-height-6);
+  }
 }
 
 .heading4xl {
-  font-size: var(--p-font-size-700);
-  line-height: var(--p-font-line-height-7);
+  font-size: var(--p-font-size-600);
+  line-height: var(--p-font-line-height-6);
+
+  @media #{$p-breakpoints-md-up} {
+    font-size: var(--p-font-size-700);
+    line-height: var(--p-font-line-height-7);
+  }
 }
 
 .bodySm {