a11y: Darker font for Settings port labels

This change introduces darker colors to Settings Crostini Port
Fowarding page to improve the background contrast ratio to reach ARIA
standards.

Screenshot: http://screen/Bp2ALHz3KFhDmzn

Bug: 1040957
Change-Id: I8e4eafd93b88d9686a5dede33cf0e01b019ca2c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2473058Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817337}

chrome/browser/resources/settings/chromeos/crostini_page/crostini_port_forwarding.html

@@ -29,7 +29,7 @@
       }
 
       .column-title {
-        color: var(--google-grey-500);
+        color: var(--google-grey-700);
       }
 
       .label-column {