Support through Rails 8 with Green CI

[jrochkind]

Drop Rails 5.2 from CI, ancient EOL and too hard to make it green.

• add webrick and rexml to gemspec, they have become standard gems, need to be specified as dependencies
• remove ancient Rails 5.2 from CI, too hard to get green
• in old EOL rails, fix concurrent-ruby to old version required to work
• fix to current caltech OAI uri for test that is really rudely using a live OAI server, come on, sigh
• CI with Rails 7.1
• fix AR setup for Rails 7.2
• CI on Rails 7.2
• allow more up to date sqlite3 in testing
• CI on Rails 8
• gemspec dev dependency allow up to AR 8