fix(a11y): add explicit labels to alert icons (#373)

Often information is conveyed by the icons of our alerts (this is an
error or warning, is this an external link) that is not conveyed
explicitly by the text. These icons are then not presentational, and
should be labeled.

## Changes

- Explicitly adds aria-labels to svgs in alerts, field level alerts, and
alert links
- removes `alt` text in favor of `aria-label` since alt text is not
universally supported on svg elements (changes to the Icon element's use
of alt text will be in a follow-up PR)

## How to test this PR

1. Are the svgs now properly labeled on alerts, field level alerts, and
alert links?

## Screenshots
![Screenshot 2024-06-28 at 4 46 36
AM](https://github.com/cfpb/design-system-react/assets/19983248/5f1d92a1-d0f8-40dd-8e79-b7ded6f5bde4)

src/components/Alert/Alert.tsx

@@ -70,7 +70,9 @@ export const Alert = ({
 
   return (
     <div className={classes} data-testid='notification' {...properties}>
-      {showIcon ? <Icon {...iconByType[status]} /> : null}
+      {showIcon ? (
+        <Icon ariaLabel={`${status} icon`} {...iconByType[status]} />
+      ) : null}
       <div className='m-notification_content'>
         {message ? (
           <p

src/components/Alert/AlertFieldLevel.tsx

@@ -44,7 +44,11 @@ export const AlertFieldLevel = ({
       className={`a-form-alert a-form-alert${AlertFieldLevelClass[status]}`}
       {...properties}
     >
-      <Icon name={MapTypeToIconName[status]} alt={status} withBg />
+      <Icon
+        ariaLabel={`${status} icon`}
+        name={MapTypeToIconName[status]}
+        withBg
+      />
       <span className='a-form-alert_text' data-testid='message'>
         {message}
       </span>

src/components/Alert/AlertLink.tsx

@@ -13,7 +13,10 @@ export const AlertLink = ({
 }: AlertLinkProperties): JSX.Element => (
   <li className='m-list_item'>
     <a className='m-list_link' href={href}>
-      {label} {isExternal ? <Icon name='external-link' /> : null}
+      {label}{' '}
+      {isExternal ? (
+        <Icon ariaLabel='external link' name='external-link' />
+      ) : null}
     </a>
   </li>
 );