fix(aria/combobox): increases autocomplete demo's placeholder text c… (#33084)

* fix(aria/combobox): increases autocomplete demo's  placeholder text contrast

Fixes a bug in Angular Aria's autocomplete demo example where
the input's placeholder text is failing text contrast at 4.4 and does not meet the a11y required 4.5:1 contrast ratio. This
provides a color variable to ::placeholder to meet the ratio
at 8.89:1.

Fixes b/477617379

* refactor(aria/combobox): fix lint errors

Fixes lint errors.

* refactor(aria/combobox): fix prefix lint errors

Author: essjay05

src/components-examples/aria/autocomplete/autocomplete.css

@@ -117,3 +117,13 @@
 .example-check-icon {
   font-size: 0.9rem;
 }
+
+/* stylelint-disable material/no-prefixes -- Provides all prefixes for ::placeholder */
+input::-webkit-input-placeholder,
+input::-moz-placeholder, /* Firefox 19+ */
+input:-ms-input-placeholder, /* IE 10+ */
+input:-moz-placeholder, /* Firefox 18- */
+input::placeholder {
+  color: var(--mat-sys-on-surface-variant);
+}
+/* stylelint-enable material/no-prefixes */