Raise Eleventy to latest stable and update its plugins

- Raise @11ty/eleventy range to ^3.1.7 (already the installed version)
- Upgrade @11ty/eleventy-plugin-rss to v3 (now ESM-only, which the
  config already uses). This fixes RFC822 dates in rss.xml: posts
  published during BST were previously timestamped an hour late.
- Upgrade @11ty/eleventy-plugin-syntaxhighlight to 5.0.2
- Remove @11ty/eleventy-upgrade-help, which was only needed for the
  v3 migration and is unused

package.json

@@ -20,10 +20,10 @@
   "author": "",
   "license": "ISC",
   "dependencies": {
-    "@11ty/eleventy": "^3.0.0",
+    "@11ty/eleventy": "^3.1.7",
     "@11ty/eleventy-fetch": "^6.0.1",
     "@11ty/eleventy-img": "^7.1.1",
-    "@11ty/eleventy-plugin-syntaxhighlight": "^5.0.0",
+    "@11ty/eleventy-plugin-syntaxhighlight": "^5.0.2",
     "@lewisdale/blog-post-cli": "^1.0.3",
     "@toycode/markdown-it-class": "^1.2.4",
     "autoprefixer": "^10.4.13",
@@ -50,8 +50,7 @@
     "tailwindcss": "^3.4.13"
   },
   "devDependencies": {
-    "@11ty/eleventy-plugin-rss": "^2.0.2",
-    "@11ty/eleventy-upgrade-help": "^3.0.1",
+    "@11ty/eleventy-plugin-rss": "^3.1.0",
     "eleventy-plugin-og-image": "^4.3.0"
   },
   "allowScripts": {