Enhance ERB rendering to trim blank lines in configuration files

[tortuetorche]

Hi,

This pull request update Kamal::Configuration so every deploy YAML (including destination-specific files like deploy.staging.yml) is rendered via ERB.new(..., trim_mode: "-"). Templates can now safely use -%> without leaving stray blank lines in the parsed config.

Example snippets:

# deploy.yml.erb
service: app
servers:
  - <%= ENV['MAIN_IP'] %>
<% if ENV['EXTRA_IP'].presence -%>
  - <%= ENV['EXTRA_IP'] %>
<% end -%>

# Without trim_mode: "-" (old behavior)
servers:
  - 1.1.1.1

  - 1.1.1.2

# With trim mode (new behavior)
servers:
  - 1.1.1.1
  - 1.1.1.2

Thank you for open-sourcing this project! 🫶

Have a good day,
Tortue Torche