refactor(mapTo): update function signature (#5102)

Reduce the number of generics used in the 'mapTo' function to 1 generic parameter. Mark the 2 generic variant as a deprecated variant

Fixes #5090

src/internal/operators/mapTo.ts

@@ -3,6 +3,10 @@ import { Subscriber } from '../Subscriber';
 import { Observable } from '../Observable';
 import { OperatorFunction } from '../types';
 
+export function mapTo<R>(value: R): OperatorFunction<any, R>;
+/** @deprecated remove in v8. Use mapTo<R>(value: R): OperatorFunction<any, R> signature instead **/
+export function mapTo<T, R>(value: R): OperatorFunction<T, R>;
+
 /**
  * Emits the given constant value on the output Observable every time the source
  * Observable emits a value.
@@ -35,8 +39,8 @@ import { OperatorFunction } from '../types';
  * @method mapTo
  * @owner Observable
  */
-export function mapTo<T, R>(value: R): OperatorFunction<T, R> {
- return (source: Observable<T>) => source.lift(new MapToOperator(value));
+export function mapTo<R>(value: R): OperatorFunction<any, R> {
+ return (source: Observable<any>) => source.lift(new MapToOperator(value));
 }
 
 class MapToOperator<T, R> implements Operator<T, R> {