Upgrade Ruby to 3.4 and fix build compatibility issues

[mmenanno]

Important

Holding this PR in draft for now as Ruby should be about to release 4.0 and I'll update this PR to use that once it is out

Summary

Upgrades Ruby and dependencies to address upcoming EOL and resolve build issues with modern Ruby versions.

Changes

Ruby & Bundler

• Ruby: Upgraded to 3.4 (Ruby 3.2 reaches EOL in ~3 months)
• Bundler: Updated to latest version

Gem Updates

• Haml: Upgraded to 7.1 (the monkey patch issue that previously blocked this has been resolved)
• General: Updated all gems to latest compatible versions

Build Fixes

Sass Deprecation

• Replaced deprecated sass gem with sassc
• Ruby Sass has reached end-of-life and should no longer be used

Markdown Renderer

Replaced Redcarpet with Kramdown to fix undefined method 'link_to' for nil error. This is a known Middleman issue with Ruby 3.x where the Redcarpet renderer context is nil.

Config changes:

# Before (Redcarpet)
set :markdown_engine, :redcarpet
set :markdown,
    fenced_code_blocks: true,
    smartypants: true,
    disable_indented_code_blocks: true,
    prettify: true,
    strikethrough: true,
    tables: true,
    with_toc_data: true,
    no_intra_emphasis: true,
    renderer: UniqueHeadCounter

# After (Kramdown)
set :markdown_engine, :kramdown
set :markdown,
    input: 'GFM',
    auto_ids: true,
    hard_wrap: false,
    smart_quotes: ['lsquo', 'rsquo', 'ldquo', 'rdquo'],
    syntax_highlighter: :rouge,
    syntax_highlighter_opts: { guess_lang: true }

Redcarpet Feature	Kramdown Equivalent
fenced_code_blocks, tables, strikethrough, no_intra_emphasis	input: 'GFM' (GitHub Flavored Markdown)
smartypants	smart_quotes
with_toc_data	auto_ids: true
UniqueHeadCounter renderer	Built-in (Kramdown auto-generates unique header IDs)
disable_indented_code_blocks	Not needed (all code uses fenced blocks)
prettify	Not needed (using Rouge syntax highlighter)

Removed unused files:

• lib/unique_head.rb - Custom Redcarpet renderer for unique header IDs (Kramdown handles this natively)
• lib/multilang.rb - Redcarpet extension that wasn't being used

JavaScript Minification

• Enabled Uglifier harmony mode to support ES6 syntax (const, let, arrow functions)

Not Updated

• middleman-sprockets: Remains pinned to 4.1.0 - verified that #2302 still affects 4.1.1 (autoprefixer sees SCSS // comments before compilation)

[jpjpjp]

Hi @mmenanno. Thank you for doing this! This PR is listed as a draft. Is it ready to be merged?

[mmenanno]

@jpjpjp I kept it in draft since the next major Ruby version comes out tomorrow on Christmas, so I figured once that is out I'll update this PR to use that so that we will maximize how much time we have till approaching EOL of a version again.