[Regression] Move css from OOBE dialog to update required screen

This change was introduced in the following CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2302639
All OOBE dialogs may not use the same color for subtitle and footer
slots. Having color in top level css is affecting some OOBE screens as
the Network screen. Hence, reverting the change for now.

Bug: 1112744
Change-Id: I00cf2fc673bfeb01513705b232fa278087c3494e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358670Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Cr-Commit-Position: refs/heads/master@{#799148}

chrome/browser/resources/chromeos/login/components/oobe_dialog.css

@@ -60,7 +60,6 @@
 }
 
 #footerContainer ::slotted(*) {
-  color: var(--oobe-subheader-text-color);
   line-height: var(--subtitle-line-height);
 }
 

chrome/browser/resources/chromeos/login/update_required_card.css

@@ -11,6 +11,10 @@
   margin-top: 20px;
 }
 
+#adminMessageContainer {
+  color: var(--oobe-subheader-text-color);
+}
+
 .bold {
   font-weight: bold;
 }