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)

db/migrate/20221204002837_remove_not_null_on_active_storage_blobs_checksum.active_storage.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+# This migration comes from active_storage (originally 20211119233751)
+class RemoveNotNullOnActiveStorageBlobsChecksum < ActiveRecord::Migration[6.0]
+  def change
+    return unless table_exists?(:active_storage_blobs)
+
+    change_column_null(:active_storage_blobs, :checksum, true)
+  end
+end

db/migrate/20221204120508_add_precision_to_timestamps.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+class AddPrecisionToTimestamps < ActiveRecord::Migration[7.0]
+  def change
+    tables = %w[
+      anomaly_notifications
+      codeharbor_links
+      comments
+      consumers
+      error_template_attributes
+      error_templates
+      events
+      execution_environments
+      exercise_collections
+      exercises
+      exercises_proxy_exercises
+      external_users
+      file_templates
+      file_types
+      files
+      internal_users
+      interventions
+      linter_check_runs
+      lti_parameters
+      proxy_exercises
+      remote_evaluation_mappings
+      request_for_comments
+      searches
+      structured_error_attributes
+      structured_errors
+      study_groups
+      submissions
+      subscriptions
+      tags
+      testruns
+      tips
+      user_exercise_feedbacks
+      user_exercise_interventions
+      user_proxy_exercise_exercises
+    ]
+
+    tables.each do |table|
+      change_column table, :created_at, :datetime, precision: 6
+      change_column table, :updated_at, :datetime, precision: 6
+    end
+
+    change_column :authentication_tokens, :expire_at, :datetime, precision: 6
+    change_column :community_solution_locks, :locked_until, :datetime, precision: 6
+    change_column :exercises, :submission_deadline, :datetime, precision: 6
+    change_column :exercises, :late_submission_deadline, :datetime, precision: 6
+    change_column :internal_users, :lock_expires_at, :datetime, precision: 6
+    change_column :internal_users, :remember_me_token_expires_at, :datetime, precision: 6
+    change_column :internal_users, :reset_password_token_expires_at, :datetime, precision: 6
+    change_column :internal_users, :reset_password_email_sent_at, :datetime, precision: 6
+    change_column :internal_users, :activation_token_expires_at, :datetime, precision: 6
+  end
+end