[ruby, ruby-rails-postgres] - Add support for debian trixie(13)

[Kaniska244]

Ref: https://github.com/devcontainers/internal/issues/283

Devcontainer Templates:

• ruby
• ruby-rails-postgres

Description of changes:

• Aims to add support for debian trixie (13)

Changelog:

• Change in Dockerfiles to add debian trixie as the default option.
• Change in devcontainer-template.json to add debian trixie image variant.
• Change in readme file.

Checklist:

• All checks are passed.