[fix] engine: re-implement mullvad leta integration

Re-writes the Mullvad Leta integration to work with the new breaking changes.

Mullvad Leta is a search engine proxy.  Currently Leta only offers text search
results not image, news or any other types of search result.  Leta acts as a
proxy to Google and Brave search results.

- Remove docstring comments regarding requiring the use of Mullvad VPN, which is
  no longer a hard requirement.

- configured two engines: ``mullvadleta`` (uses google) and
  ``mullvadleta brave`` (uses brave)

- since leta may not provide up-to-date search results, both search engines are
  disabled by default.

.. hint::

   Leta caches each search for up to 30 days.  For example, if you use search
   terms like ``news``, contrary to your intention you'll get very old results!

Co-authored-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Grant Lanham <contact@grantlanham.com>

searx/settings.yml

@@ -1420,14 +1420,21 @@ engines:
       require_api_key: false
       results: JSON
 
-  # read https://docs.searxng.org/dev/engines/online/mullvad_leta.html
-  # - name: mullvadleta
-  #   engine: mullvad_leta
-  #   leta_engine: google # choose one of the following: google, brave
-  #   use_cache: true  # Only 100 non-cache searches per day, suggested only for private instances
-  #   search_url: https://leta.mullvad.net
-  #   categories: [general, web]
-  #   shortcut: ml
+  # https://docs.searxng.org/dev/engines/online/mullvad_leta.html
+  - name: mullvadleta
+    engine: mullvad_leta
+    disabled: true
+    leta_engine: google
+    categories: [general, web]
+    shortcut: ml
+
+  - name: mullvadleta brave
+    engine: mullvad_leta
+    network: mullvadleta
+    disabled: true
+    leta_engine: brave
+    categories: [general, web]
+    shortcut: mlb
 
   - name: odysee
     engine: odysee