Document that mix test --dry-run was introduced in 1.20.0 (#15484)

Author: zachallaun

lib/mix/lib/mix/tasks/test.ex

@@ -121,7 +121,7 @@ defmodule Mix.Tasks.Test do
 
     * `--cover` - runs coverage tool. See "Coverage" section below
 
-    * `--dry-run` *(since v1.19.0)* - prints which tests would be run based on current options,
+    * `--dry-run` *(since v1.20.0)* - prints which tests would be run based on current options,
       but does not actually run any tests. This combines with all other options
       like `--stale`, `--only`, `--exclude`, and so on.