[KeyboardKey] Consolidate se23 logic and styles (#10010)

### WHY are these changes introduced?

Fixes #9941

### WHAT is this pull request doing?

Consolidates se23 beta styles for `KeyboardKey`.

### How to 🎩

Compare:

* This PR's
[Storybook](https://5d559397bae39100201eedc1-xmguxoqurw.chromatic.com/?path=/story/all-components-keyboardkey--all)
* [Production
Storybook](https://storybook.polaris.shopify.com/?path=/story/all-components-keyboardkey--all&globals=polarisSummerEditions2023:true)

.changeset/tall-chicken-repeat.md

@@ -2,4 +2,4 @@
 '@shopify/polaris': major
 ---
 
-Removed Summer Editions experimental styles and code for the following components: `Avatar`, `AccountConnection`, `ActionList`, `ActionMenu`, `Autocomplete`, `Breadcrumbs`, `Text`
+Removed Summer Editions experimental styles and code for the following components: `Avatar`, `AccountConnection`, `ActionList`, `ActionMenu`, `Autocomplete`, `Breadcrumbs`, `Text`, `KeyboardKey`

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

@@ -7,41 +7,25 @@ $key-base-dimension: 28px;
   display: inline-flex;
   justify-content: center;
   margin: 0 var(--p-space-05) var(--p-space-05);
+  margin-bottom: 0;
   padding: 0 var(--p-space-2);
-  background: var(--p-color-bg-inset);
-  // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
-  box-shadow: 0 var(--p-space-025) 0 rgba(0, 0, 0, 0.15);
+  background: var(--p-color-bg-secondary-experimental);
   border-radius: var(--p-border-radius-1);
-  color: var(--p-color-text);
+  color: var(--p-color-text-subdued);
   font-size: var(--p-font-size-100);
   font-weight: var(--p-font-weight-medium);
   font-family: var(--p-font-family-sans);
   line-height: $key-base-dimension;
   text-align: center;
   min-width: $key-base-dimension;
   user-select: none;
-
-  #{$se23} & {
-    background: var(--p-color-bg-secondary-experimental);
-    border-radius: var(--p-border-radius-1);
-    color: var(--p-color-text-subdued);
-    box-shadow: none;
-    margin-bottom: 0;
-  }
 }
 
 .small {
-  background: var(--p-color-bg-app-selected);
-  border-radius: var(--p-border-radius-05);
   box-shadow: none;
-  line-height: unset;
-  padding: 0 var(--p-space-1);
+  line-height: var(--p-font-size-200);
+  padding: var(--p-space-05) var(--p-space-1);
   font-size: var(--p-font-size-75);
   height: var(--p-space-5);
   min-width: var(--p-space-5);
-
-  #{$se23} & {
-    line-height: var(--p-font-size-200);
-    padding: var(--p-space-05) var(--p-space-1);
-  }
 }