chore: upgrade Ruby 2.6 -> 3.3 and Jekyll 3.8 -> 4.3

[botbeech]

Summary

Moves the stack off EOL Ruby 2.6 / Jekyll 3.8 onto a currently maintained baseline. Site source (posts, layouts, includes, custom plugins) is untouched — change set is entirely build/CI/config.

What changes

Component	Before	After
Ruby	2.6 (EOL 2022-03)	3.3 (pinned via new .ruby-version)
Bundler	1.13.6 in lock / 1.17.3 in CI	ships with Ruby 3.3 — no pin
Jekyll	3.8.5	~> 4.3 (resolves to 4.4.1)
kramdown	1.17	~> 2.4 + new kramdown-parser-gfm ~> 1.1
rouge	3.3	~> 4.2
jekyll-feed	0.12	~> 0.17
jekyll-sitemap	1.3	~> 1.4
jekyll-paginate	1.1	kept at 1.1 (custom _plugins/ depend on Jekyll::Paginate::Pager)
webrick	stdlib	added as explicit gem (Ruby 3.0 removed it from stdlib)

Files touched

• Gemfile, Gemfile.lock (regenerated) — version bumps above.
• .ruby-version (new) — 3.3.6; ruby/setup-ruby@v1 and local tooling read this.
• Dockerfile — ruby:2.6 -> ruby:3.3-slim; drop BUNDLER_VERSION arg (bundler ships with Ruby 3); add build-essential for native gem extensions.
• docker-compose.yml — drop the stale BUNDLER_VERSION: "1.17.2" build arg.
• _config.yml — explicit kramdown: { input: GFM, syntax_highlighter: rouge } so tables / strikethrough / fenced code keep working after kramdown 2's GFM split.
• .github/workflows/pages.yml:
  • Drop the ruby-version: '2.6' + bundler: '1.17.3' pins; ruby/setup-ruby@v1 now reads .ruby-version.
  • Add a new check job on pull_request (no if: guard, no deploy, no artifact upload). Gives PR authors and forks a build smoke test. The existing build + deploy jobs remain guarded with github.repository == 'botbie/blog' and now also github.event_name != 'pull_request' so PRs don't race against the deploy pipeline.

What intentionally does NOT change

• _plugins/jekyll-paginate-{authors,tags}.rb — both subclass Jekyll::Paginate::Pager and use stable Jekyll::Page / Jekyll::Generator APIs that are unchanged in Jekyll 4.4. No edits needed.
• _layouts/*, _includes/* — paginator field names (paginator.previous_page, .next_page, .page, .total_pages, .total_posts, .posts) are unchanged between jekyll-paginate 1.1 and Jekyll 4.
• Post frontmatter, permalinks, URL structure — byte-identical output directory layout.
• No Sass pipeline changes (the site uses plain CSS — verified).

Local validation

Bundled install + bundle exec jekyll build on Ruby 3.1.2 (via apt, as 3.3 wasn't available in the sandbox image — but all dependency constraints are >= 3.1, so 3.3 on CI runners is a strict superset). Verified:

• /index.html + /page/2/index.html render.
• /author/<username>/index.html for every user in _data/authors.yml.
• /author/<username>/page/N/index.html pagination tails (up to /page/4/ for prolific authors).
• /tag/<slug>/index.html + /tag/<slug>/page/N/index.html for every tag.
• A sample post (/2016/06/10/bitcoin-blockchain.html) renders with correct title, description meta, and Vietnamese content preserved.
• /feed.xml, /sitemap.xml, /CNAME all present.
• 133 HTML pages + 3 XML files, 19 MB total output.

No errors. One warning (see below).

Known pre-existing issue surfaced

Build now logs:

Conflict: The following destination is shared by multiple files.
          /_site/tag/ctf/index.html
          - _layouts/tag.html
          - _layouts/tag.html

Root cause: _plugins/jekyll-paginate-tags.rb slugifies via tag.downcase.strip.gsub(' ', '-').gsub(/[^\w-]/, ''), so the tags "ctf" (2 posts) and "CTF" (1 post) both map to /tag/ctf/. Same pattern affects "WebAssembly", "Java", "String" (capitalized variants with no lowercase collision today, safe so far). This is not introduced by the upgrade — the same logic would have produced the same conflict on Jekyll 3. Worth a follow-up PR (either normalize tags at post-read time, or add a de-dup pass in the generator); not fixing here to keep this PR focused.

Follow-ups (out of scope)

• Fix the tag case-collision above.
• Consider migrating _plugins/jekyll-paginate-* to jekyll-paginate-v2 so we can drop the jekyll-paginate 1.x dependency eventually.
• Enable Dependabot for bundler + github-actions ecosystems now that the floor is modern.