Upgrade Ruby to 2.7.3 (#16004)

* Upgrade Ruby to 2.7.3

https://www.ruby-lang.org/en/news/2021/04/05/ruby-2-7-3-released/
includes security fixes to
- CVE-2021-28965: XML round-trip vulnerability in REXML
- CVE-2021-28966: Path traversal in Tempfile on Windows

* Update rexml to 3.2.5

https://www.ruby-lang.org/en/news/2021/04/05/xml-round-trip-vulnerability-in-rexml-cve-2021-28965/

Dockerfile

@@ -26,7 +26,7 @@ RUN ARCH= && \
 	mv node-v$NODE_VER-linux-$ARCH /opt/node
 
 # Install Ruby
-ENV RUBY_VER="2.7.2"
+ENV RUBY_VER="2.7.3"
 RUN apt-get update && \
   apt-get install -y --no-install-recommends build-essential \
     bison libyaml-dev libgdbm-dev libreadline-dev libjemalloc-dev \