frontend-plugin-api: remove type support for multiple attachment points

This is a follow-up to the deprecation in #32521, completely removing
the Array variant from the ExtensionDefinitionAttachTo type.

The runtime still supports multiple attachment points for backward
compatibility, but new code will receive type errors.

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

packages/frontend-app-api/src/wiring/createSpecializedApp.test.tsx

@@ -648,12 +648,13 @@ describe('createSpecializedApp', () => {
               output: [coreExtensionData.reactElement],
               factory: () => [coreExtensionData.reactElement(<div />)],
             }),
+            // Test backward compatibility - runtime still supports multiple attachment points
             createExtension({
               name: 'cloned',
               attachTo: [
                 { id: 'test/a', input: 'children' },
                 { id: 'test/b', input: 'children' },
-              ],
+              ] as any,
               output: [coreExtensionData.reactElement],
               factory: () => [coreExtensionData.reactElement(<div />)],
             }),