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.
