fix(alerts) - Update team selector dropdown for alerts (#63897)

## Description
Fix spacing on the `Unassigned` item in the dropdown when selecting the
Team for an alert.

Fixes: https://github.com/getsentry/team-enterprise/issues/44

### Before
![Screenshot 2024-01-25 at 1 03
54 PM](https://github.com/getsentry/sentry/assets/1569818/5a2804f6-8655-4c8b-9d2b-6be62af533eb)

### After
![Screenshot 2024-01-25 at 1 17
28 PM](https://github.com/getsentry/sentry/assets/1569818/9fe207be-faa3-4b01-9431-7e4ef78af325)

static/app/views/alerts/list/rules/row.tsx

@@ -259,8 +259,8 @@ function RuleListRow({
     value: '',
     label: () => (
       <MenuItemWrapper>
-        <StyledIconUser size="md" />
-        {t('Unassigned')}
+        <PaddedIconUser size="lg" />
+        <Label>{t('Unassigned')}</Label>
       </MenuItemWrapper>
     ),
     searchKey: 'unassigned',
@@ -308,7 +308,7 @@ function RuleListRow({
     />
   ) : (
     <Tooltip isHoverable skipWrapper title={t('Unassigned')}>
-      <StyledIconUser size="md" color="gray400" />
+      <PaddedIconUser size="lg" color="gray400" />
     </Tooltip>
   );
 
@@ -495,9 +495,8 @@ const StyledChevron = styled(IconChevron)`
   margin-left: ${space(1)};
 `;
 
-const StyledIconUser = styled(IconUser)`
-  /* We need this to center with Avatar */
-  margin-right: 2px;
+const PaddedIconUser = styled(IconUser)`
+  padding: ${space(0.25)};
 `;
 
 const IconContainer = styled('div')`