chore(ui): change /devtest to /-/demo (#11019)

It has always been largely used for showcasing UI elements but that name didn't work too well for it.

Testing:
Some of existing tests depend on these pages, making it redundant to create extra tests.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11019
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

modules/testlogger/testlogger.go

@@ -367,7 +367,7 @@ var ignoredErrorMessage = []string{
 	// Test_CmdForgejo_Actions
 	`DB: No dedicated replica host defined; falling back to primary DSN for replica connections`,
 
-	// TestDevtestErrorpages
+	// TestDemoErrorPages
 	`ErrorPage() [E] Example error: Example error`,
 }