fix: bump Ruby and outdated deps, drop dead sass gem

Author: nicholaswmin

Dockerfile

@@ -1,4 +1,4 @@
-FROM ruby:2.6-slim
+FROM ruby:3.3-slim
 
 WORKDIR /srv/slate
 

Gemfile

@@ -1,15 +1,14 @@
-ruby '>= 2.5'
+ruby '>= 3.2'
 source 'https://rubygems.org'
 
 # Middleman
 gem 'middleman', :github => 'middleman/middleman', :branch => '4.x'
 gem 'middleman-syntax', '~> 3.2'
 gem 'middleman-autoprefixer', '~> 2.7'
 gem 'middleman-sprockets', '~> 4.1'
-gem 'rouge', '~> 3.21'
-gem 'redcarpet', '~> 3.5.0'
-gem 'nokogiri', '~> 1.11.0'
-gem 'sass'
+gem 'rouge', '~> 4.4'
+gem 'redcarpet', '~> 3.6'
+gem 'nokogiri', '~> 1.16'
 gem 'webrick'
-gem "rack-contrib", "~> 2.3"
-gem "puma", "~> 5.3"
+gem "rack-contrib", "~> 2.5"
+gem "puma", "~> 6.5"