Release Notes: Build PDF with a theme that supports extended unicode

This requires using a newer version of asciidoctor-pdf.  So in the
GitHub CI job that builds the release notes on ubuntu-22.04, use gem
to install the latest version over what the package manager installs.

Author: quark17

.github/workflows/install_dependencies_releasenotes_ubuntu.sh

@@ -4,3 +4,6 @@ apt-get update
 apt-get install -y \
   asciidoctor \
   ruby-asciidoctor-pdf
+
+# Update asciidoctor-pdf to the latest version
+gem install asciidoctor-pdf

release/Makefile

@@ -67,7 +67,7 @@ ReleaseNotes.html: ReleaseNotes.adoc
 	asciidoctor ReleaseNotes.adoc
 
 ReleaseNotes.pdf: ReleaseNotes.adoc
-	asciidoctor-pdf ReleaseNotes.adoc
+	asciidoctor-pdf --theme=default-with-font-fallbacks ReleaseNotes.adoc
 
 # -----