Declare missing indirect deps for Ruby 3.4 compat.

These default gems are slated for removal from Ruby 3.4, but upstream
gems that depend on them haven't updated their gemspecs yet (or hadn't
at the time of release of the we're stuck on for now, at least).

This somewhat helps forward-compatibility with Ruby 3.4.

govuk_tech_docs.gemspec

@@ -36,7 +36,10 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = ">= 3.1.0"
 
   spec.add_dependency "autoprefixer-rails", "~> 10.2"
+  spec.add_dependency "base64" # TODO: remove once middleman-sprockets declares this itself.
+  spec.add_dependency "bigdecimal" # TODO: remove once activesupport declares this itself.
   spec.add_dependency "chronic", "~> 0.10.2"
+  spec.add_dependency "csv" # TODO: remove once tilt declares this itself.
   spec.add_dependency "haml", "~> 6.0"
   spec.add_dependency "middleman", "~> 4.0"
   spec.add_dependency "middleman-autoprefixer", "~> 2.10"
@@ -45,6 +48,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "middleman-search-gds"
   spec.add_dependency "middleman-sprockets", "~> 4.0.0"
   spec.add_dependency "middleman-syntax", "~> 3.4"
+  spec.add_dependency "mutex_m" # TODO: remove once activesupport declares this itself.
   spec.add_dependency "nokogiri"
   spec.add_dependency "openapi3_parser", "~> 0.9.0"
   spec.add_dependency "redcarpet", "~> 3.6"