Fix the variable regexp pattern on web page (#27161)

same as (https://github.com/go-gitea/gitea/pull/26910)

templates/shared/variables/variable_list.tmpl

@@ -73,7 +73,7 @@
 					name="name"
 					id="dialog-variable-name"
 					value="{{.name}}"
-					pattern="^[a-zA-Z_][a-zA-Z0-9_]*$"
+					pattern="^(?!GITEA_|GITHUB_)[a-zA-Z_][a-zA-Z0-9_]*$"
 					placeholder="{{.locale.Tr "secrets.creation.name_placeholder"}}"
 				>
 			</div>