Fix loading all Tasks with namespace collision

After upgrading from Rails 7 to 7.1, we started seeing an error when
trying to load the Maintenance Task UI:

```
undefined method `instance_of?` for #<BasicObject:... >
```

We ended up identifying the BasicObject as PRIMARY_KEY_NOT_SET, which
was added to Active Record in 7.1. The reason for attempting to load
this constant was that we have a model in our app named Maintenance
which inherits from ActiveRecord::Base, and so all of the constants
below ActiveRecord::Base get loaded by load_constants. (This clearly
isn't a common case, nor something I would recommend other applications
do).

To fix this, we can use the Rails application's loader to load all of
the application's tasks instead of traversing the constant tree. This
approach has multiple benefits:
- it will only load constants managed by the application's loader (which
  is the intention anyway)
- load_constants will be faster in production because the loader will
  recognize that the namespace has already been loaded and so will not
  have to traverse the tree

Since `#eager_load_namespace` was introduced in Zeitwerk 2.6.2 but the
minimum required by Rails is 2.6, the higher minimum version was added
to the gemspec as well.

Gemfile.lock

@@ -7,6 +7,7 @@ PATH
       activerecord (>= 6.0)
       job-iteration (>= 1.3.6)
       railties (>= 6.0)
+      zeitwerk (>= 2.6.2)
 
 GEM
   remote: https://rubygems.org/

app/models/maintenance_tasks/task.rb

@@ -187,13 +187,7 @@ def load_constants
         namespace = MaintenanceTasks.tasks_module.safe_constantize
         return unless namespace
 
-        load_const = lambda do |root|
-          root.constants.each do |name|
-            object = root.const_get(name)
-            load_const.call(object) if object.instance_of?(Module)
-          end
-        end
-        load_const.call(namespace)
+        Rails.autoloaders.main.eager_load_namespace(namespace)
       end
     end
 

maintenance_tasks.gemspec

@@ -26,4 +26,5 @@ Gem::Specification.new do |spec|
   spec.add_dependency("activerecord", ">= 6.0")
   spec.add_dependency("job-iteration", ">= 1.3.6")
   spec.add_dependency("railties", ">= 6.0")
+  spec.add_dependency("zeitwerk", ">= 2.6.2")
 end