Remove Astro 6 banner - #13990

Changed pages include:

• 404.html - view
• ar/404/index.html - view
• ar/astro-courses/index.html - view
• ar/develop-and-build/index.html - view
• ar/editor-setup/index.html - view
• ar/getting-started/index.html - view
• ar/install-and-setup/index.html - view
• ar/recipes/index.html - view
• ar/upgrade-astro/index.html - view
• de/404/index.html - view

There are more changes not included in the list