MDL-69391 theme: Darken course selector text colour

Use $gray-600 for a darker text colour in the course select box.
This also makes the colour consistent with the view selector which
also uses $gray-600.

theme/boost/scss/moodle/calendar.scss

@@ -455,5 +455,5 @@ table.calendartable caption {
 }
 
 .cal_courses_flt {
-    color: #868e96;
+    color: $gray-600;
 }

theme/boost/style/moodle.css

@@ -12986,7 +12986,7 @@ table.calendartable caption {
     margin: 0; }
 
 .cal_courses_flt {
-  color: #868e96; }
+  color: #6c757d; }
 
 .content-bank-container .cb-content-wrapper {
   padding: 0.5rem;

theme/classic/style/moodle.css

@@ -13201,7 +13201,7 @@ table.calendartable caption {
     margin: 0; }
 
 .cal_courses_flt {
-  color: #868e96; }
+  color: #6c757d; }
 
 .content-bank-container .cb-content-wrapper {
   padding: 0.5rem;