email: Recommend Thunderbird Mobile instead of K-9 Mail

- Thunderbird Mobile has been built on K-9 mail and is the future of the app[1].

Links:

1)
https://blog.thunderbird.net/2022/06/revealed-thunderbird-on-android-plans-k9/

Tests:

- Go to email app and expand the clients information. Verify the app store links
for Thunderbird Mobile.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/email/manifest.py

@@ -34,17 +34,17 @@ clients = [
     },
     {
         'name':
-            _('K-9 Mail'),
+            _('Thunderbird Mobile'),
         'platforms': [{
             'type': 'store',
             'os': 'android',
             'store_name': 'f-droid',
-            'url': store_url('f-droid', 'com.fsck.k9')
+            'url': store_url('f-droid', 'net.thunderbird.android')
         }, {
             'type': 'store',
             'os': 'android',
             'store_name': 'google-play',
-            'url': store_url('google-play', 'com.fsck.k9')
+            'url': store_url('google-play', 'net.thunderbird.android')
         }]
     },
     {