fix(material/table): increase specificity on sticky selector

The sticky styles were switched to a class in #19823 which has a very low specificity.
As a result, there's a high chance of it being overwritten by some user styles.

These changes nest the selector so it's a bit harder to override

Fixes #20558.

Author: crisbeto

src/material-experimental/mdc-table/table.scss

@@ -6,7 +6,9 @@
 @include mdc-data-table-core-styles($query: $mat-base-styles-without-animation-query);
 @include mat-private-table-flex-styles();
 
-.mat-mdc-table-sticky {
+// Note that this uses a nested selector in order to increase the specificity and to ensure
+// that the sticky styles apply over any others that may have been added afterwards. (See #19823).
+.mat-mdc-table .mat-mdc-table-sticky {
   @include vendor-prefixes.position-sticky;
 }
 

src/material/table/table.scss

@@ -52,7 +52,9 @@ th.mat-header-cell, td.mat-cell, td.mat-footer-cell {
   }
 }
 
-.mat-table-sticky {
+// Note that this uses a nested selector in order to increase the specificity and to ensure
+// that the sticky styles apply over any others that may have been added afterwards. (See #19823).
+.mat-table .mat-table-sticky {
   @include vendor-prefixes.position-sticky;
 }