Update header.html

Issue: #953 Updated banner message with date of migration

_includes/header.html

@@ -50,13 +50,10 @@
   </div>
   <div class="usa-overlay"></div>
 </div>
-<!-- Added 06-22-2023 to inform users of website and content move -->
+<!-- Updated 07-10-2023 - with specific update schedule -->
 <div class="usa-alert usa-alert--warning" style="margin-top: 0px;">
   <div class="usa-alert__body">
     <h4 class="usa-alert__heading">Important Update:</h4>
-    <p class="usa-alert__text">
-      In the near future, this website and its content will move to <a class="usa-link usa-lin`k--external" href="https://www.idmanagement.gov" target="_self" aria-label="IDManagement.gov website">IDManagement.gov</a>.<br>
-      Once this move is complete, all future traffic to this site will be redirected to it's new location.   
-    </p>
+    <p class="usa-alert__text">On <strong>August 4th, 2023</strong>, <i>playbooks.idmanagement.gov</i> will move to <a class="usa-link usa-lin`k--external" href="https://www.idmanagement.gov" target="_self" aria-label="IDManagement.gov website">IDManagement.gov</a>. A preview of the new site is available at <a class="usa-link usa-lin`k--external" href="https://qa.idmanagement.gov/" target="_self" aria-label="A preview of the new IDManagement.gov site">https://qa.idmanagement.gov/</a>. Some links may break with this migration. Please update any bookmarks to the new location.</p>
   </div>
 </div>