feat(store): remove getMockStore in favor of createMockStore (#3835)

BREAKING CHANGES:

The `getMockStore` function is removed in favor of `createMockStore`

BEFORE:

```ts
import { getMockStore } from '@ngrx/store/testing';
const mockStore = getMockStore();
```

AFTER:

```ts
import { createMockStore } from '@ngrx/store/testing';
const mockStore = createMockStore();
```

modules/store/testing/src/testing.ts

@@ -126,14 +126,6 @@ function mockStoreFactory<T>(
   );
 }
 
-/**
- * @deprecated This function is deprecated in favor of `createMockStore`.
- * If you are looking to get an existing mock store, you probably want to use `TestBed.inject(MockStore)` instead.
- *
- * For more info see: https://github.com/ngrx/platform/issues/3781
- */
-export const getMockStore = createMockStore;
-
 /**
  * @description
  * Creates mock store with all necessary dependencies outside of the `TestBed`.
@@ -160,7 +152,9 @@ export const getMockStore = createMockStore;
  * });
  * ```
  */
-export function createMockStore<T>(config: MockStoreConfig<T> = {}): MockStore<T> {
+export function createMockStore<T>(
+  config: MockStoreConfig<T> = {}
+): MockStore<T> {
   const injector = Injector.create({ providers: provideMockStore(config) });
   return injector.get(MockStore);
 }