fix(input-group): textarea missing disabled styling

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/components/input/_input-group-component.scss

@@ -169,6 +169,10 @@
         @include e(input) {
             @extend %form-group-input--disabled !optional;
         }
+
+        @include e(textarea) {
+            @extend %form-group-textarea--disabled !optional;
+        }
     }
 
     @include m(cosy) {

projects/igniteui-angular/src/lib/core/styles/components/input/_input-group-theme.scss

@@ -979,6 +979,14 @@
         }
     }
 
+    %form-group-textarea--disabled {
+        color: --var($theme, 'disabled-text-color') !important;
+
+        &::placeholder {
+            color: --var($theme, 'disabled-placeholder-color');
+        }
+    }
+
     %form-group-textarea--cosy {
         min-height: rem(82px, map-get($base-scale-size, 'cosy'));
         margin-#{$right}: -#{rem(16px, map-get($base-scale-size, 'cosy'))};