Reestablish support for new Solargraph versions

[apiology]

• [regression] Fix interface change breaking solargraph-rails castwide/solargraph#940
• [breaking] Reimplement global conventions castwide/solargraph#877
• Run specs from plugin projects configured against current code castwide/solargraph#892
• [regression] Add back bundler/require support for solargraph-rails castwide/solargraph#941 (needed for activestorage support without an explicit require)
• Add a published Solargraph release to CI matrix and get it to pass
• Remove pointers in Gemfile to branches