[MM-61677]: Update heading level (#29784)

* [MA-51]: Update heading level

* [MA-51]: Fixed E2E tests

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Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/components/post_priority/post_priority_picker.tsx

@@ -42,7 +42,7 @@ const PersistentNotificationsIcon = styled(BellRingOutlineIcon)`
     fill: rgba(var(--center-channel-color-rgb), 0.75);
 `;
 
-const Header = styled.h4`
+const Header = styled.h2`
     align-items: center;
     display: flex;
     gap: 8px;