fix(material-experimental/mdc-card): reset native header margin (#22550)

`mat-card` supports using native header elements for the title and subtitle, but doing so throws off the alignment, because we weren't clearing the user agent margin.

(cherry picked from commit f88023c)

Author: crisbeto

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

@@ -22,6 +22,10 @@ $mat-card-default-padding: 16px !default;
   // Apply default padding for a text content region. Omit any bottom padding because we assume
   // this region will be followed by another region that includes top padding.
   padding: $mat-card-default-padding $mat-card-default-padding 0;
+
+  // Titles and subtitles can be set on native header elements which come with
+  // their own margin. Clear it since the spacing is done using `padding`.
+  margin: 0;
 }
 
 // Header section at the top of a card. MDC does not have a pre-made header section for cards.