Fix: update aria-label from “Mainbar” to “Main navigation bar” for clarity

Replaced the aria-label `Mainbar` used for the main navigation with
`Main navigation bar` to improve clarity and accessibility for
screen reader users, as requested in ticket #43945.

Author: ZallaxDev

lang/ilias_en.lang

@@ -4650,7 +4650,7 @@ common#:#mails#:#Mails
 common#:#mails_at_the_ilias_installation#:#You received %1$s new mails at the ILIAS-Installation %2$s
 common#:#mails_pl#:#Mail(s)
 common#:#main_menu#:#Main Menu
-common#:#mainbar_aria_label#:#Mainbar
+common#:#mainbar_aria_label#:#Main navigation bar
 common#:#mainbar_more_label#:#More
 common#:#manage_members#:#Manage Members
 common#:#marked_entries#:#Marked Entries