fix(material/radio): assistive technology announcing button as invalid

Fixes that some screen readers were reading out "required, invalid data" for radio buttons that were marked as required. The problem was triggered by us using the `required` attribute. Switching it to `aria-required` seems to resolve the issue.

Fixes #30761.

Author: crisbeto

src/material/radio/radio.html

@@ -8,7 +8,7 @@
            [disabled]="disabled && !disabledInteractive"
            [attr.name]="name"
            [attr.value]="value"
-           [required]="required"
+           [attr.aria-required]="required ? true : null"
            [attr.aria-label]="ariaLabel"
            [attr.aria-labelledby]="ariaLabelledby"
            [attr.aria-describedby]="ariaDescribedby"

src/material/radio/testing/radio-harness.ts

@@ -217,7 +217,7 @@ export class MatRadioButtonHarness extends ComponentHarness {
 
   /** Whether the radio-button is required. */
   async isRequired(): Promise<boolean> {
-    const required = (await this._input()).getAttribute('required');
+    const required = (await this._input()).getAttribute('aria-required');
     return coerceBooleanProperty(await required);
   }