[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>

e2e-tests/playwright/support/ui/components/channels/message_priority.ts

@@ -25,7 +25,7 @@ export default class MessagePriority {
 
         // Priority dialog elements
         this.priorityDialog = container.page().getByRole('dialog');
-        this.dialogHeader = this.priorityDialog.locator('h4.modal-title');
+        this.dialogHeader = this.priorityDialog.locator('h2.modal-title');
     }
 
     async clickPriorityIcon() {

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;