dropdown-item has wrong border-radius

[TomONeill]

Prerequisites

• I have searched for duplicate or closed issues
• I have validated any HTML to avoid common problems
• I have read the contributing guidelines

Describe the issue

In PR #27703, border-radius was added to fix the overflow. Then in #28127 there was an additional fix for when the dropdown padding is 0.

But the :first-child and :last-child selectors aren't right. They'll always be true since a .dropdown-item is always wrapped in a <li>.

Reduced test cases

$dropdown-padding-y: 0;
$dropdown-border-width: 0;
$dropdown-item-padding-y: 0.625rem;
$dropdown-item-padding-x: 0.75rem;

<ul class="dropdown-menu show">
  <li><a class="dropdown-item" href="#">Regular link</a></li>
  <li><a class="dropdown-item active" href="#" aria-current="true">Active link</a></li>
  <li><a class="dropdown-item" href="#">Another link</a></li>
</ul>

What operating system(s) are you seeing the problem on?

Windows

What browser(s) are you seeing the problem on?

Chrome

What version of Bootstrap are you using?

v5.1.3