added rel noopener to fix sonar security issue (#922)

Dfe.ManageFreeSchoolProjects/Dfe.ManageFreeSchoolProjects/Pages/Errors/_PageNotFound.cshtml

@@ -10,7 +10,7 @@
 			If you pasted the web address, check you copied the entire address.
 		</p>
 		<p class="govuk-body">
-			If the web address is correct or you selected a link or button, <a href="@Configuration["SupportLink"]" class="govuk-link" target="_blank">get help with a Regions Group System (opens in a new tab).</a>
+			If the web address is correct or you selected a link or button, <a href="@Configuration["SupportLink"]" class="govuk-link" target="_blank" rel="noopener">get help with a Regions Group System (opens in a new tab).</a>
 		</p>
 	</div>
 </div>