Attempted deployment fix by requiring default gems

When moving to Ruby 3.1 some parts of the standard library were moved to
default gems[0][1]. While we're on Rails 6.x they need to be included
in the gem file, as discussed in this this GitHub issue[2].

Once we upgrade to Rails 7 these requirments can be dropped.

[0] https://bugs.ruby-lang.org/issues/17873
[1] https://stdgems.org/#default-gems-ruby-301
[2] mikel/mail#1439 (comment)

Gemfile

@@ -28,6 +28,8 @@ gem "sentry-rails"
 gem "sentry-ruby"
 gem "webpacker"
 
+gem "net-imap", require: false
+gem "net-pop", require: false
 gem "net-smtp", require: false
 
 group :development, :test do

Gemfile.lock

@@ -222,6 +222,14 @@ GEM
     msgpack (1.5.2)
     multi_json (1.15.0)
     multipart-post (2.1.1)
+    net-imap (0.2.3)
+      digest
+      net-protocol
+      strscan
+    net-pop (0.1.1)
+      digest
+      net-protocol
+      timeout
     net-protocol (0.1.3)
       timeout
     net-smtp (0.3.1)
@@ -390,6 +398,7 @@ GEM
       actionpack (>= 5.2)
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
+    strscan (3.0.3)
     thor (1.2.1)
     thread_safe (0.3.6)
     timeout (0.3.0)
@@ -458,6 +467,8 @@ DEPENDENCIES
   lograge (>= 0.11.2)
   logstash-event
   mail-notify
+  net-imap
+  net-pop
   net-smtp
   pagy
   pg (>= 0.18, < 2.0)