Use SHA256 to encrypt messages and cookies

Note that enabling this options means all encrypted messages and cookies
generated the application become invalid, so we're adding a cookie
rotator in order to keep sessions from expiring when upgrading the
application, as recommended in the "Upgrading Ruby on Rails" guideline
[1].

Since we haven't seen any Consul Democracy applications using encrypted
messages and these messages become invalid with this change, we're also
removing the pre-Rails 5.2 encryption to authenticate messages
(AES-256-CBC) and switching to the default one since Rails 5.2
(AES-256-GCM). Since the configured encryption is used by the cookie
rotator initializer (through the ActiveSupport::MessageEncryptor.key_len
method), at first I thought this might affect the cookie rotator, but it
doesn't: upgrading works as expected, and existing sessions are still
active.

I'm adding a comment to remove the initializer once all cookies have
been migrated. I've added "Rails 7.1" in the comment because we usually
check for these comments when upgrading Rails, but we rarely check for
them when after releasing new versions of Consul Democracy.

[1] https://guides.rubyonrails.org/v7.0/upgrading_ruby_on_rails.html#key-generator-digest-class-changing-to-use-sha256

.rubocop.yml

@@ -163,6 +163,7 @@ Layout/LineLength:
     - "config/environments/staging.rb"
     - "config/initializers/devise.rb"
     - "config/initializers/backtrace_silencers.rb"
+    - "config/initializers/cookie_rotator.rb"
     - "db/migrate/*create_delayed_jobs.rb"
     - "db/migrate/*create_active_storage_variant_records.active_storage.rb"
     - "app/models/budget/stats.rb"