fix(alert): Sets autoCloseDuration to "medium" by default (#7157)

**Related Issue:** #6363

## Summary

fix(alert): Sets autoCloseDuration to "medium" by default. #6363

@jcfranco ~~can we have linting to prevent having any conditional
defaults? If not, we should maybe update conventions to say not to do
this~~ Lets discuss how we can properly document and test this pattern.

packages/calcite-components/src/components/alert/alert.e2e.ts

@@ -1,9 +1,18 @@
 import { E2EElement, E2EPage, newE2EPage } from "@stencil/core/testing";
 import { html } from "../../../support/formatting";
-import { accessible, hidden, HYDRATED_ATTR, renders, t9n } from "../../tests/commonTests";
+import { accessible, defaults, hidden, HYDRATED_ATTR, renders, t9n } from "../../tests/commonTests";
 import { getElementXY } from "../../tests/utils";
 import { CSS, DURATIONS } from "./resources";
 
+describe("defaults", () => {
+  defaults("calcite-alert", [
+    {
+      propertyName: "autoCloseDuration",
+      defaultValue: "medium"
+    }
+  ]);
+});
+
 describe("calcite-alert", () => {
   const alertContent = `
     <div slot="title">Title Text</div>

packages/calcite-components/src/components/alert/alert.tsx

@@ -100,7 +100,7 @@ export class Alert implements OpenCloseComponent, LoadableComponent, T9nComponen
   @Prop({ reflect: true }) autoClose = false;
 
   /** Specifies the duration before the component automatically closes (only use with `autoClose`). */
-  @Prop({ reflect: true }) autoCloseDuration: AlertDuration = this.autoClose ? "medium" : null;
+  @Prop({ reflect: true }) autoCloseDuration: AlertDuration = "medium";
 
   /** Specifies the kind of the component (will apply to top border and icon). */
   @Prop({ reflect: true }) kind: Extract<