Use base-browser.ftl instead of languageNotification.ftl

We've renaming the Base Browser Fluent file.

For $reasons we kept calling it languageNotification.ftl, but now we've finally renamed it into base-browser.ftl (see tor-browser#41984 (closed)).

While doing that, we've also moved it to the base-browser branch, so we can also delete references to basebrowser-newidentityftl