Add support for Ruby 2.6; drop 2.3
Add support for Ruby 2.6; drop 2.3

Adds support for Ruby 2.6, which is in the build matrix as of #3565.

Ruby 2.3 falls out of its security maintenance phase on the 31st of this
month. That will likely predate the 3.0.0 release, meaning support should be
dropped in that release. This removes it in anticipation of that.

samvera/hyrax,samvera/hyrax,samvera/hyrax,samvera/hyrax