Update Ruby to 3.1.2

Reason for Change
=================
* Stay current
* [Release notes](https://www.ruby-lang.org/en/news/2022/04/12/ruby-3-1-2-released/)

Changes
=======
* Update `.ruby-version` file
* Update bundler version
* Update buildpack to `heroku-22`
* Update CircleCI config
* Add `net-smtp` gem explicitly because of [this]

[this]: https://stackoverflow.com/questions/70500220/rails-7-ruby-3-1-loaderror-cannot-load-such-file-net-smtp

Author: adarsh

.circleci/config.yml

@@ -11,7 +11,7 @@ jobs:
   build_and_test:
     working_directory: ~/cfp_app
     docker:
-      - image: cimg/ruby:3.0.4-browsers
+      - image: cimg/ruby:3.1.2-browsers
         environment:
           PGHOST: localhost
           PGUSER: cfp_app

.ruby-version

@@ -1 +1 @@
-3.0.4
+3.1.2

Gemfile

@@ -10,6 +10,13 @@ gem 'rails', '~> 6.1.4'
 gem 'mimemagic', github: 'mimemagicrb/mimemagic', ref: '01f92d86d15d85cfd0f20dabd025dcbd36a8a60f'
 gem 'mime-types-data'
 gem 'mime-types'
+gem 'rexml'
+gem 'matrix'
+
+# Required until Rails 7 - https://github.com/mikel/mail/pull/1472#issuecomment-1165161541
+gem 'net-smtp', require: false
+# gem 'net-imap', require: false
+# gem 'net-pop', require: false
 
 gem 'pg'
 
@@ -20,7 +27,6 @@ gem 'jquery-datatables-rails'
 gem 'jquery-rails'
 gem 'jquery-ui-rails'
 gem 'rails-assets-momentjs', source: 'https://rails-assets.org'
-gem 'rexml'
 gem 'sassc-rails'
 gem 'selectize-rails'
 gem 'uglifier', '>= 1.3.0'

Gemfile.lock

@@ -156,6 +156,7 @@ GEM
       responders
       warden (~> 1.2.3)
     diff-lcs (1.4.4)
+    digest (3.1.0)
     dotenv (2.7.6)
     dotenv-rails (2.7.6)
       dotenv (= 2.7.6)
@@ -278,6 +279,7 @@ GEM
     mail (2.7.1)
       mini_mime (>= 0.1.1)
     marcel (1.0.2)
+    matrix (0.4.2)
     method_source (1.0.0)
     mime-types (3.3.1)
       mime-types-data (~> 3.2015)
@@ -291,6 +293,12 @@ GEM
     multi_xml (0.6.0)
     multipart-post (2.1.1)
     nenv (0.3.0)
+    net-protocol (0.1.3)
+      timeout
+    net-smtp (0.3.1)
+      digest
+      net-protocol
+      timeout
     nio4r (2.5.8)
     nokogiri (1.13.6)
       mini_portile2 (~> 2.8.0)
@@ -474,6 +482,7 @@ GEM
     thor (1.2.1)
     tilt (2.0.10)
     timecop (0.9.4)
+    timeout (0.3.0)
     tinymce-rails (5.10.3)
       railties (>= 3.1.1)
     tzinfo (2.0.4)
@@ -543,9 +552,11 @@ DEPENDENCIES
   jquery-rails
   jquery-ui-rails
   launchy
+  matrix
   mime-types
   mime-types-data
   mimemagic!
+  net-smtp
   nokogiri
   omniauth-github
   omniauth-rails_csrf_protection (~> 1.0)
@@ -584,7 +595,7 @@ DEPENDENCIES
   webpacker
 
 RUBY VERSION
-   ruby 3.0.4p208
+   ruby 3.1.2p20
 
 BUNDLED WITH
-   2.3.11
+   2.3.17

app.json

@@ -1,5 +1,5 @@
 {
-  "stack": "heroku-20",
+  "stack": "heroku-22",
   "name": "CFP-App",
   "description": "Conference call for proposal management application by Ruby Central",
   "keywords": [