bump ruby version to 2.7.3

This has some security fixes, though none that should impact us. Mostly
I want to make sure the bumping process works with CI (it would be nice
if we could read the .ruby-version or Gemfile from the CI code; I think
it's possible but non-trivial).

Author: peff

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
     - name: ruby setup
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.7.2
+        ruby-version: 2.7.3
         bundler-cache: true
 
     - name: rails setup

.ruby-version

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

Gemfile

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 source "https://rubygems.org"
-ruby "2.7.2"
+ruby "2.7.3"
 
 gem "rails", "~> 6.0"
 

Gemfile.lock

@@ -356,7 +356,7 @@ DEPENDENCIES
   yajl-ruby
 
 RUBY VERSION
-   ruby 2.7.2p137
+   ruby 2.7.3p183
 
 BUNDLED WITH
    2.1.4