Implement text color changes across components

The commit involves modifications in various components to change 'text-gray-500' to 'text-muted-foreground'. The purpose of this change is to create uniformity in text colors across the application, enhancing the user interface's overall look and making color scheme tweaks easier in the future.

packages/ui/src/makerkit/image-upload-input.tsx

@@ -148,7 +148,7 @@ export const ImageUploadInput = forwardRef<React.ElementRef<'input'>, Props>(
         <div className={'flex items-center space-x-4'}>
           <div className={'flex'}>
             <If condition={!state.image}>
-              <UploadCloud className={'dark:text-dark-500 h-5 text-gray-500'} />
+              <UploadCloud className={'h-5 text-muted-foreground'} />
             </If>
 
             <If condition={state.image}>

packages/ui/src/makerkit/stepper.tsx

@@ -98,7 +98,7 @@ function getClassNameBuilder() {
       {
         variant: 'default',
         selected: false,
-        className: 'text-gray-400 dark:text-gray-500',
+        className: 'text-muted-foreground',
       },
       {
         variant: 'default',
@@ -108,7 +108,7 @@ function getClassNameBuilder() {
       {
         variant: 'default',
         selected: false,
-        className: 'bg-gray-300 dark:bg-gray-800',
+        className: 'bg-muted',
       },
       {
         variant: 'numbers',
@@ -118,7 +118,7 @@ function getClassNameBuilder() {
       {
         variant: 'numbers',
         selected: false,
-        className: 'text-gray-400 dark:text-gray-500',
+        className: 'text-muted-foreground',
       },
     ],
     defaultVariants: {
@@ -135,7 +135,7 @@ function StepDivider({
   selected: boolean;
 }>) {
   const spanClassName = cn('font-medium text-sm', {
-    ['text-gray-400 dark:text-gray-500']: !selected,
+    ['text-muted-foreground']: !selected,
     ['text-primary']: selected,
   });