Replace N/A with - everywhere (#24474)

Followup to https://github.com/go-gitea/gitea/pull/24427.

Reasoning is that `N/A` is specific to english while `-` is
language-neutral and does not need translation.

Before:
<img width="891" alt="Screenshot 2023-05-01 at 20 58 20"
src="https://user-images.githubusercontent.com/115237/235511592-8a36d0f2-34ff-4dbe-b642-67c0ade644fe.png">

After:
<img width="901" alt="Screenshot 2023-05-01 at 20 59 59"
src="https://user-images.githubusercontent.com/115237/235511594-d49f6d09-92e8-4e99-be7b-2a37f5d24129.png">

web_src/js/features/repo-settings.js

@@ -32,7 +32,7 @@ export function initRepoSettingsCollaboration() {
           if ($item) {
             $dropdown.dropdown('set selected', $dropdown.attr('data-last-value'));
           } else {
-            $text.text('(N/A)'); // prevent from misleading users when the access mode is undefined
+            $text.text('(none)'); // prevent from misleading users when the access mode is undefined
           }
         }, 0);
       }