Use the newer serialize signature if available

On 7.1 serialize properly split `coder` and `type` arguments
instead of taking a single positional argument and try to figure
out which is which.

While we're at it, we can explictly define we're using YAML here
instead of relying on the default that is now configurable.

app/models/maintenance_tasks/run.rb

@@ -46,8 +46,13 @@ class Run < ApplicationRecord
 
     attr_readonly :task_name
 
-    serialize :backtrace
-    serialize :arguments, JSON
+    if Rails.gem_version >= Gem::Version.new("7.1.alpha")
+      serialize :backtrace, coder: YAML
+      serialize :arguments, coder: JSON
+    else
+      serialize :backtrace
+      serialize :arguments, JSON
+    end
 
     scope :active, -> { where(status: ACTIVE_STATUSES) }
     scope :completed, -> { where(status: COMPLETED_STATUSES) }