[mod] normalize variable name for the max number of results per request (#6131)

[mod] normalize variable name for the max number of results per request

In the past, we have used different names for the variable that specifies the
maximum number of hits in the outgoing request.

- ``page_size``
- ``number_of_results``
- ``nb_per_page``

Since *page_size* is the most accurate term and is also used in the XPath
engines, all other engines are adjusted accordingly within this
patch .. documentation adjusted accordingly.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/settings.yml

@@ -1391,7 +1391,6 @@ engines:
     engine: metacpan
     shortcut: cpan
     disabled: true
-    number_of_results: 20
 
   # https://docs.searxng.org/dev/engines/offline/search-indexer-engines.html#module-searx.engines.meilisearch
   # - name: meilisearch