Disable IMAP auth check in Rake task when Microsoft Graph is configured

MailRoom supports two inbox modes: IMAP and Microsoft Graph. The former
uses the IMAP protocol, and the latter uses a REST API to retrieve
e-mail.

When we added support for Microsoft Graph, we did not account for this
in the `gitlab:incoming_email:check` Rake task.

For now, disable the IMAP check if is it not configured. We may want to
consider adding a Microsoft Graph-specific check, but MailRoom will exit
if the Microsoft Graph credentials are not configured properly.

Relates to
https://gitlab.com/gitlab-org/gitlab/-/issues/214900#note_656345704

config/initializers/1_settings.rb

@@ -251,6 +251,7 @@ Settings.gitlab_ci['url']                 ||= Settings.__send__(:build_gitlab_ci
 #
 Settings['incoming_email'] ||= Settingslogic.new({})
 Settings.incoming_email['enabled'] = false if Settings.incoming_email['enabled'].nil?
+Settings.incoming_email['inbox_method'] ||= 'imap'
 
 #
 # Service desk email

lib/system_check/incoming_email_check.rb

@@ -7,9 +7,11 @@ module SystemCheck
 
     def multi_check
       if Gitlab.config.incoming_email.enabled
-        checks = [
-          SystemCheck::IncomingEmail::ImapAuthenticationCheck
-        ]
+        checks = []
+
+        if Gitlab.config.incoming_email.inbox_method == 'imap'
+          checks << SystemCheck::IncomingEmail::ImapAuthenticationCheck
+        end
 
         if Rails.env.production?
           checks << SystemCheck::IncomingEmail::InitdConfiguredCheck

spec/lib/system_check/incoming_email_check_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe SystemCheck::IncomingEmailCheck do
+  before do
+    allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('production'))
+  end
+
+  describe '#multi_check' do
+    context 'when incoming e-mail is disabled' do
+      before do
+        stub_incoming_email_setting(enabled: false)
+      end
+
+      it 'does not run any checks' do
+        expect(SystemCheck).not_to receive(:run)
+
+        subject.multi_check
+      end
+    end
+
+    context 'when incoming e-mail is enabled for IMAP' do
+      before do
+        stub_incoming_email_setting(enabled: true)
+      end
+
+      it 'runs IMAP and mailroom checks' do
+        expect(SystemCheck).to receive(:run).with('Reply by email', [
+          SystemCheck::IncomingEmail::ImapAuthenticationCheck,
+          SystemCheck::IncomingEmail::InitdConfiguredCheck,
+          SystemCheck::IncomingEmail::MailRoomRunningCheck
+        ])
+
+        subject.multi_check
+      end
+    end
+
+    context 'when incoming e-mail is enabled for Microsoft Graph' do
+      before do
+        stub_incoming_email_setting(enabled: true, inbox_method: 'microsoft_graph')
+      end
+
+      it 'runs mailroom checks' do
+        expect(SystemCheck).to receive(:run).with('Reply by email', [
+          SystemCheck::IncomingEmail::InitdConfiguredCheck,
+          SystemCheck::IncomingEmail::MailRoomRunningCheck
+        ])
+
+        subject.multi_check
+      end
+    end
+  end
+end