Fix uninitialized constant error on Puma boot

Before, using the default `puma/config.rb` generated by `rails new`,
Puma would fail to boot in production with this error:

```
./config/puma.rb:16:in `block in _load_from': uninitialized constant Puma::DSL::Concurrent (NameError)
	from ./config/puma.rb:16:in `fetch'
	from ./config/puma.rb:16:in `_load_from'
```

This was because `Concurrent.physical_processor_count` was being
referenced before the `Concurrent` constant was initialized.

Fix by requiring `concurrent-ruby` just before the `Concurrent` constant
is needed.

Fixes #49323

railties/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Require `concurrent-ruby` in `config/puma.rb` so that Puma can boot in
+    production when `WEB_CONCURRENCY` is not explicitly specified.
+
+    Fixes #49323.
+
+    *Matt Brictson*
+
 *   Raise error when generating attribute with dangerous name.
 
     The following will now raise an error as `save` and `hash` are already

railties/lib/rails/generators/rails/app/templates/config/puma.rb.tt

@@ -13,6 +13,7 @@ threads min_threads_count, max_threads_count
 
 # Specifies that the worker count should equal the number of processors in production.
 if ENV["RAILS_ENV"] == "production"
+  require "concurrent-ruby"
   worker_count = Integer(ENV.fetch("WEB_CONCURRENCY") { Concurrent.physical_processor_count })
   workers worker_count if worker_count > 1
 end