Implemented feedback to changesets, theme provider, and core-imports test

- Removed validation logic for `appTheme.Provider` as type checking should be enough
- Replaced `createApp` with `createSpecializedApp` in `core-imports.test.ts`
- Upped changeset level from patch to minor and made minor verbiage tweaks

Signed-off-by: Colton Padden <colton.padden@fastmail.com>

packages/codemods/src/tests/core-imports.test.ts

@@ -45,8 +45,9 @@ return something()
   it('should refactor imports', () => {
     const input = `
 /* COPYRIGHT: ME */
-import { Button as MyButton, createApiRef } from '@backstage/core';
+import { Button as MyButton, createApiRef, createSpecializedApp } from '@backstage/core';
 
+const app = createSpecializedApp();
 const apiRef = createApiRef();
 const button = <MyButton />
 `;
@@ -56,7 +57,9 @@ const button = <MyButton />
 import { Button as MyButton } from '@backstage/core-components';
 
 import { createApiRef } from '@backstage/core-plugin-api';
+import { createSpecializedApp } from '@backstage/core-app-api';
 
+const app = createSpecializedApp();
 const apiRef = createApiRef();
 const button = <MyButton />
 `;