Restore more specific CSS for paragraph block custom color classes.

This extra specificity was removed in #13025, but should be restored because other blocks (buttons, pullquotes, etc),  use the `has-background` and `has-text-color` classes. We don't want the styles here to interfere.

Using a `p` selector here, rather than something like `.wp-block-paragraph` so that this gets picked up correctly on the front-end as well.

packages/block-library/src/paragraph/style.scss

@@ -35,10 +35,10 @@
 	padding-top: $block-padding;
 }
 
-.has-background {
+p.has-background {
 	padding: 20px 30px;
 }
 
-.has-text-color a {
+p.has-text-color a {
 	color: inherit;
 }