Use estimated_count instead of size on unfiltered criterias with Mongoid >= 7.2.0

[kalilz4485]

With mongoid 7.2.0 comes a change that may bring performances issue to kaminari

The .count method will really count documents when used without filters, instead of using the estimate count stored in the collection https://github.com/mongodb/mongoid/pull/4808/files

kaminari-mongoid uses size which is an alias for count, my approach is to override it to use estimate_count when we can

I didn't add any spec since it's not changing any expected result nor fixing anything