[ResourceList][IndexTable] Fix border-radius and padding bugs

[chloerice]

WHY are these changes introduced?

Fixes #8047

WHAT is this pull request doing?

This PR fixes a few small CSS bugs that shipped with the BulkActions redesign:

• Fixed header of IndexTable with bulkActions not having border radius in select mode
• Fixed header of selectable ResourceList not having border radius
• Fixed selectable ResourceList with no bulkActions having extra bottom padding in select mode
• Fixed last ResourceItem in a selectable ResourceList with bulkActions having a border radius when in select mode

Before	After

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import React from 'react';
import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
    </Page>
  );
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide

[github-actions]

size-limit report 📦

Path	Size
polaris-react-cjs	221.02 KB (+0.02% 🔺)
polaris-react-esm	140.53 KB (+0.03% 🔺)
polaris-react-esnext	196.63 KB (+0.03% 🔺)
polaris-react-css	42.6 KB (+0.03% 🔺)