docs: remove all deprecated @stable jsdoc tags (#23210)

These are no longer needed as stable docs are computed as those that
do not have `@experimental` or `@deprecated` tags.

PR Close #23210

tools/public_api_guard/core/testing.d.ts

@@ -1,10 +1,8 @@
-/** @stable */
 export declare function async(fn: Function): (done: any) => any;
 
 /** @experimental */
 export declare function cleanupDocument(): void;
 
-/** @stable */
 export declare class ComponentFixture<T> {
     changeDetectorRef: ChangeDetectorRef;
     componentInstance: T;
@@ -47,7 +45,6 @@ export declare function flushMicrotasks(): void;
 /** @experimental */
 export declare function getTestBed(): TestBed;
 
-/** @stable */
 export declare function inject(tokens: any[], fn: Function): () => any;
 
 /** @experimental */
@@ -66,7 +63,6 @@ export declare type MetadataOverride<T> = {
 /** @experimental */
 export declare function resetFakeAsyncZone(): void;
 
-/** @stable */
 export declare class TestBed implements Injector {
     ngModule: Type<any> | Type<any>[];
     platform: PlatformRef;