MDL-79063 theme_boost: Match .disabled font colour with .text-muted

* For consistency
* The font colour for .text-muted meets a contrast ratio of 4.69:1
which meets WCAG 2.1 Level AA success criterion for minimum contrast.

theme/boost/scss/moodle/core.scss

@@ -2076,7 +2076,7 @@ a.disabled {
     text-decoration: none;
     cursor: default;
     font-style: italic;
-    color: #808080;
+    color: $text-muted;
 }
 
 body.lockscroll {

theme/boost/style/moodle.css

@@ -24945,7 +24945,7 @@ a.disabled {
   text-decoration: none;
   cursor: default;
   font-style: italic;
-  color: #808080;
+  color: #6a737b;
 }
 
 body.lockscroll {

theme/classic/style/moodle.css

@@ -24945,7 +24945,7 @@ a.disabled {
   text-decoration: none;
   cursor: default;
   font-style: italic;
-  color: #808080;
+  color: #6a737b;
 }
 
 body.lockscroll {