Support blacklight 8, rails 8, propshaft

[chrisrlc]

• Drops support for blacklight 7, adds support for blacklight 8
• Drops support for rails 7, adds support for rails 8
• Drops support for sprockets, adds support for propshaft with importmaps or package.json-based bundler like jsbundling-rails
  • Install generator supports cssbundling-rails, but blacklight generator blocks installer from generating with something like dartsass-rails (if no application.bootstrap.scss present, blacklight generator installs sprockets) - can still be used without cssbundling, but generator may not work properly
• Moves js to stimulus controller
• Requires importing leaflet assets instead of maintaining in vendor/
• Fixes constraints display (bug also present in v1.2) by using bl8 FilterField pattern for coordinate search paramsc
• Adds solr docker container option
• Updates ci matrix
• Updates version in version.rb, new package.json, and README to 2.0.0 - I'm guessing that's what we'd want?

[jcoyne]

This change is too big to review as a single pull request. Can you please separate this into more manageable chunks? Ideally a pull request is no more than 500 lines changed. Otherwise it's likely we'll miss something.

[ebenenglish]

@jcoyne given that this project hasn't any significant development in years, it's going to be very tough to untangle the various dependency updates and removal of outdated asset pipelines/architecture from other code changes.

It's a big PR, but I'm more than happy to review.