[ticket/12244] Rename link to Extensions Manager in the ACP

PHPBB3-12244
[ticket/12244] Rename extensions manager tab

PHPBB3-12244
Merge remote-tracking branch 'vsephpbb/ticket/12244' into develop

* vsephpbb/ticket/12244:
  [ticket/12244] Rename extensions manager tab
  [ticket/12244] Rename link to Extensions Manager in the ACP