Deprecate HeaderPage in favor of Header in @backstage/ui (#33188)

* Deprecate HeaderPage in favor of Header in @backstage/ui

Rename the HeaderPage component to Header, keeping HeaderPage as a
deprecated alias for backwards compatibility. Also deprecate
HeaderPageProps, HeaderPageOwnProps, HeaderPageBreadcrumb, and
HeaderPageDefinition with new Header* equivalents. Update all internal
usages, stories, and docs-ui documentation to use the new names.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

* Rename HeaderPage files and directories to Header

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

* Add changeset for mui-to-bui Header rename

Document the plugin release impact of switching the MUI to BUI theme converter page to the renamed Header component.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

---------

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

plugins/mui-to-bui/src/components/BuiThemerPage/BuiThemerPage.tsx

@@ -16,7 +16,7 @@
 
 import { useApi } from '@backstage/core-plugin-api';
 import { appThemeApiRef } from '@backstage/core-plugin-api';
-import { Box, Card, Container, Flex, HeaderPage, Text } from '@backstage/ui';
+import { Box, Card, Container, Flex, Header, Text } from '@backstage/ui';
 import { ThemeContent } from './ThemeContent';
 import { MuiThemeExtractor } from './MuiThemeExtractor';
 
@@ -26,7 +26,7 @@ export const BuiThemerPage = () => {
 
   return (
     <Container>
-      <HeaderPage title="BUI Theme Converter" />
+      <Header title="BUI Theme Converter" />
       <Box m="4">
         <Text variant="body-medium" color="secondary">
           Convert your MUI v5 theme into BUI CSS variables. Pick a theme to view