MDL-76415 core_privacy: Fixed ${var} string interpolation deprecations.

Since PHP 8.2, placing the dollar sign outside the curly brace is deprecated when
the expression inside the braces resolves to a variable or an expression.

privacy/classes/tests/provider_testcase.php

@@ -100,7 +100,7 @@ public function export_context_data_for_user(int $userid, \context $context, str
      * @return  string
      */
     protected function get_provider_classname($component) {
-        $classname = "\\${component}\\privacy\\provider";
+        $classname = "\\{$component}\\privacy\\provider";
 
         if (!class_exists($classname)) {
             throw new \coding_exception("{$component} does not implement any provider");