Use kramdown-rfc from gem

Author: rfc2822

.github/workflows/build-rfc.yml

@@ -1,7 +1,7 @@
 name: Build and publish document
-on:
-  push:
-    branches: [main]
+on: push
+# push:
+#   branches: [main]
 
 # Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
 permissions:
@@ -20,7 +20,7 @@ jobs:
           key: webdav-push.mkd
 
       - name: Install packages
-        run: sudo apt-get install --no-install-recommends -y ruby-kramdown-rfc2629 xml2rfc
+        run: sudo gem install kramdown-rfc && sudo apt-get install --no-install-recommends -y xml2rfc
 
       - name: Prepare output directory
         run: mkdir build