fix(forms): accept number length in length validators (#32057)

Both `MinLengthValidator` and `MaxLengthValidator` accepted only string inputs for the length required, which throws with Ivy and `fullTemplateTypeCheck` enabled:

    <!-- min = 2 in the component -->
    <input [minlength]="min">

with:

    Type 'number' is not assignable to type 'string | undefined'

This relaxes the accepted type to `string | number` to avoid breakage when developers switch to Ivy and fTTC.

PR Close #32057

tools/public_api_guard/forms/forms.d.ts

@@ -330,14 +330,14 @@ export declare class FormsModule {
 }
 
 export declare class MaxLengthValidator implements Validator, OnChanges {
-    maxlength: string;
+    maxlength: string | number;
     ngOnChanges(changes: SimpleChanges): void;
     registerOnValidatorChange(fn: () => void): void;
     validate(control: AbstractControl): ValidationErrors | null;
 }
 
 export declare class MinLengthValidator implements Validator, OnChanges {
-    minlength: string;
+    minlength: string | number;
     ngOnChanges(changes: SimpleChanges): void;
     registerOnValidatorChange(fn: () => void): void;
     validate(control: AbstractControl): ValidationErrors | null;