Copy Puma systemd unit template from capistrano3-puma

This commit copies the default puma.service.erb template from the
capistrano3-puma gem into lib/capistrano/templates. This allows us to
customize the generated systemd unit file during deploy.

Note that we are also removing the `:puma_conf` variable from `config/deploy.rb`,
as the new ExecStart line in the systemd template (based on capistrano3-puma 6.0.0)
does not rely on a separate Puma config file. The command now directly invokes:

  ExecStart=<%= expanded_bundle_command %> exec puma -e <%= fetch(:puma_env) %>

This replaces the older format used in 5.2.0:

  ExecStart=<%= expanded_bundle_command %> exec --keep-file-descriptors puma -C <%= fetch(:puma_conf) %>

which required explicitly setting the Puma config path.

config/deploy.rb

@@ -49,7 +49,6 @@ set :fnm_setup_command, -> do
 set :fnm_install_node_command, -> { "#{fetch(:fnm_setup_command)} && fnm use --install-if-missing" }
 set :fnm_map_bins, %w[node npm rake yarn]
 
-set :puma_conf, "#{release_path}/config/puma/#{fetch(:rails_env)}.rb"
 set :puma_systemctl_user, :user
 set :puma_enable_socket_service, true
 set :puma_service_unit_env_vars, ["EXECJS_RUNTIME=Disabled"]