Upgrade to Eleventy 2.x - #60

Bump @11ty/eleventy to ^3.1.6. The only breaking change hit was
stricter permalink extension validation (new in 3.x): blog/feed.njk
intentionally writes an extensionless /blog/feed file (Content-Type
is set via the _headers Netlify rule instead), which now needs
eleventyAllowMissingExtension: true in its front matter to avoid a
fatal build error.

Verified: npm run build-ci passes (908 files copied / 564 written,
4 tests green) and npm run serve confirms the dev server, 404 page,
and live-reload script still behave the same as on 2.x.

v3 is ESM-only, so requiring it from this CJS .eleventy.js now returns
the module namespace object rather than the plugin function directly —
unwrap it via `.default`.

v3 also removed the rssDate and rssLastUpdatedDate filters (they threw
with a message pointing at their replacements). Replaced every call
site across feed/feed.njk, feed/json.njk, feed/podcast.njk, and
blog/feed.njk with the new dateToRfc3339 (Atom, JSON Feed) /
dateToRfc822 (RSS 2.0) + getNewestCollectionItemDate filters, matching
each feed format's actual date spec.

Note: the podcast RSS feed's <pubDate>/<lastBuildDate> now render as
proper RFC822 (e.g. "Mon, 20 Mar 2023 00:00:00 +0000") instead of the
1.0.2 baseline's ISO8601 output, which wasn't RSS 2.0-spec-compliant
to begin with — a content diff from baseline, but a correctness fix,
not a regression. Atom and JSON Feed output are byte-identical to
baseline.

Verified: npm run build-ci passes (908 files copied / 564 written,
4 tests green); manually checked date formatting in all four feed
outputs.

No API changes needed — still CJS, addPlugin() signature unchanged.

Verified: npm run build-ci passes (908 files copied / 564 written,
4 tests green); diffed a post's syntax-highlighted code block output
against the pre-upgrade baseline, byte-identical.

No API changes needed — still CJS, addPlugin() and the
eleventyNavigation filter signature both unchanged.

Verified: npm run build-ci passes (908 files copied / 564 written,
4 tests green); diffed the rendered <nav> block against the
pre-upgrade baseline, byte-identical.

Records the handful of things the original pre-work assessment didn't
anticipate (permalink extension validation, eleventy-plugin-rss going
ESM-only and dropping rssDate/rssLastUpdatedDate) so future readers
don't have to reconstruct them from the commit history.