Fix missing NonNull on Observable.map (#7302)

* Fix missing NonNull on Observable.map

I noticed this while testing the upcoming Kotlin 1.5.30-M1 (which has improved support for type-use nullability annotations) https://youtrack.jetbrains.com/issue/KT-47833. This makes `at least match Single, but `Maybe.map` is also missing this. I suspect there are others too.

* Update src/main/java/io/reactivex/rxjava3/core/Observable.java

Author: ZacSweers

src/main/java/io/reactivex/rxjava3/core/Observable.java

@@ -10371,7 +10371,7 @@ public final <R> Observable<R> lift(@NonNull ObservableOperator<? extends R, ? s
     @CheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
     @NonNull
-    public final <R> Observable<R> map(@NonNull Function<? super T, ? extends R> mapper) {
+    public final <@NonNull R> Observable<R> map(@NonNull Function<? super T, ? extends R> mapper) {
         Objects.requireNonNull(mapper, "mapper is null");
         return RxJavaPlugins.onAssembly(new ObservableMap<>(this, mapper));
     }