changed helper text to control-label class (#25969)

* changed helper text to control-label class

* removed the help-text class

* added "helping-text" className

* Updated helping-text color

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Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/sass/components/_inputs.scss

@@ -3,7 +3,7 @@
 .help-text {
     display: block;
     margin: 10px 0 0;
-    color: $dark-gray;
+    color: rgba(var(--center-channel-color-rgb), 0.75);
 
     ul,
     ol {