INSPIRE Atom feeds / Fix link to atom search endpoint

[josegar74]

Follow up of #6395

The URL in the ATOM Feeds OpenSearchDescription.xml document pointed to /htmlsearch. This service is not defined in GeoNetwork 4.x and also in GeoNetwork 3.12.x was redirecting to the application search results page. What looks not correct as the endpoint is to return ATOM Feeds.

Updated to use the correct endpoint.

Test case (with local feeds):

1. Enable the INSPIRE setting
2. Create an iso19139 service metadata
3. Request the OpenSearchDescription.xml document:

http://localhost:8080/geonetwork/srv/opensearch/OpenSearchDescription.xml?uuid=SERVICEUUID

Without the change:

http://localhost:8080/geonetwork/srv/en/opensearch/htmlsearch?q={searchTerms?}

Endpoint fails as doesn't exist

With the change:

http://localhost:8080/geonetwork/opensearch/en/search?q={searchTerms?}

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