Revert default Ruby version to 2.7.2 (#16154)

Ruby 2.7.3 introduced a new bug with Resolv::DNS, which we heavily use within
Mastodon: https://bugs.ruby-lang.org/issues/17781

Ruby 2.7.3 also included security fixes for two CVEs, but those do not seem
to apply to Mastodon:
https://github.com/tootsuite/mastodon/pull/16004#issuecomment-815125025

.ruby-version

@@ -1 +1 @@
-2.7.3
+2.7.2

Dockerfile

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