Move to Ruby 3

This includes updating our validation yaml loading to allow parsing
date values in the kit versions file, which was previously implicitly
allowed but now needs to be explicit.

.ruby-version

@@ -1 +1 @@
-2.7.3
+3.3.0

Dockerfile

@@ -1,4 +1,4 @@
-FROM ruby:2.7
+FROM ruby:3.3
 
 ENV GLOBAL_GEMS=true
 

Rakefile

@@ -1,3 +1,4 @@
+require 'date'
 require 'fileutils'
 require 'set'
 require 'yaml'
@@ -44,7 +45,7 @@ end
 task :build_spellings => [:build, :spelling_dependencies]
 
 task :validate_kit_versions do
-  data = YAML.load_file('_data/kit_versions.yml')
+  data = YAML.load_file('_data/kit_versions.yml', permitted_classes: [Date])
   data.each do |entry|
     actual = entry.keys.to_set
     expected = ['version', 'released', 'link', 'changelog'].to_set