Upgrade to Rails 7.0 and apply new framework defaults

* Remove `send_stream` method pulled in before upgrading Rails
* Remove spring, it is no longer included by default for new apps
* Remove deprecated options from environments
* Remove old asset paths and workarounds no longer needed
* Remove unnecessary `OAUTH_10_SUPPORT` const, LTI still uses OAuth 1.0
* Dump schema with new defaults (and specify precision for timestamps where needed)

Gemfile

@@ -34,7 +34,7 @@ gem 'prometheus_exporter'
 gem 'pry-byebug'
 gem 'puma'
 gem 'pundit'
-gem 'rails', '~> 6.1.7'
+gem 'rails', '~> 7.0.4'
 gem 'rails_admin', '< 4.0.0'
 gem 'rails-i18n'
 gem 'rails-timeago'
@@ -76,10 +76,6 @@ group :development, :staging do
   gem 'rubocop-rspec'
 end
 
-group :development, :test, :staging do
-  gem 'spring'
-end
-
 group :test do
   gem 'autotest' # required by autotest-rails
   gem 'autotest-rails'