MDL-81428 theme_boost: Match .disabled opacity with .text-muted

theme/boost/scss/moodle/core.scss

@@ -1974,6 +1974,10 @@ a.disabled {
     color: $text-muted;
 }
 
+a.btn.disabled {
+    opacity: 1;
+}
+
 body.lockscroll {
     height: 100%;
     overflow: hidden;

theme/boost/style/moodle.css

@@ -24971,6 +24971,10 @@ a.disabled {
   color: #6a737b;
 }
 
+a.btn.disabled {
+  opacity: 1;
+}
+
 body.lockscroll {
   height: 100%;
   overflow: hidden;

theme/classic/style/moodle.css

@@ -24971,6 +24971,10 @@ a.disabled {
   color: #6a737b;
 }
 
+a.btn.disabled {
+  opacity: 1;
+}
+
 body.lockscroll {
   height: 100%;
   overflow: hidden;