Define Puma log paths for consistency with installer

capistrano3-puma v6.0.0 updated the defaults for puma_access_log and
puma_error_log to use a single file based on puma_env, like:

> set_if_empty :puma_access_log, -> { File.join(shared_path, 'log', "#{fetch(:puma_env)}.log") }
> set_if_empty :puma_error_log, -> { File.join(shared_path, 'log', "#{fetch(:puma_env)}.log") }

However, our installer expect:
- puma_access.log
- puma_error.log

To keep the existing behavior aligned with the installer, we define
the Puma log paths in config/deploy.rb

config/deploy.rb

@@ -54,6 +54,8 @@ set :puma_enable_socket_service, true
 set :puma_service_unit_env_vars, ["EXECJS_RUNTIME=Disabled"]
 set :puma_service_unit_name, -> { "puma_#{fetch(:application)}_#{fetch(:stage)}" }
 set :puma_bind, "unix://#{shared_path}/tmp/sockets/puma.sock"
+set :puma_access_log, -> { File.join(shared_path, "log", "puma_access.log") }
+set :puma_error_log, -> { File.join(shared_path, "log", "puma_error.log") }
 
 set :delayed_job_workers, 2
 set :delayed_job_roles, :background