_previous_next_doc.html.erb deprecation in 7.x should use updated component name

[sandbergja]

Right now on the release-7.x branch, _previous_next_doc.html.erb includes the following deprecation notice:

The partial _previous_next_doc.html.erb will be removed in 8.0. Render Blacklight::SearchContextComponent instead.

This is outdated advice, since Blacklight::SearchContextComponent was renamed to Blacklight::SearchContext::ServerItemPaginationComponent in commit 9bd3ecb.

We should update the deprecation message. It would also be nice to backport that component to release-7.x without removing SearchContextComponent, so that Blacklight 7 sites can start using ServerItemPaginationComponent rather than SearchContextComponent.