DEV: Convert email_in_min_trust to groups (#24515)

This change converts the `email_in_min_trust` site setting to
`email_in_allowed_groups`.

See: https://meta.discourse.org/t/283408

- Hides the old setting
- Adds the new site setting
- Add a deprecation warning
- Updates to use the new setting
- Adds a migration to fill in the new setting if the old setting was
  changed
- Adds an entry to the site_setting.keywords section
- Updates tests to account for the new change

After a couple of months we will remove the
`email_in_min_trust` setting entirely.

Internal ref: /t/115696

config/locales/server.en.yml

@@ -2140,6 +2140,8 @@ en:
     log_mail_processing_failures: "Log all email processing failures to <a href='%{base_path}/logs' target='_blank'>/logs</a>"
     email_in: "Allow users to post new topics via email. After enabling this setting, you will be able to configure incoming email addresses for groups and categories."
     email_in_min_trust: "The minimum trust level a user needs to have to be allowed to post new topics via email."
+    email_in_allowed_groups: "Groups that are allowed to post new topics via email."
+
     email_in_authserv_id: "The identifier of the service doing authentication checks on incoming emails. See <a href='https://meta.discourse.org/t/134358'>https://meta.discourse.org/t/134358</a> for instructions on how to configure this."
     email_in_spam_header: "The email header to detect spam."
 
@@ -2547,6 +2549,7 @@ en:
       shared_drafts_allowed_groups: "shared_drafts_min_trust_level"
       approve_unless_allowed_groups: "approve_unless_trust_level"
       approve_new_topics_unless_allowed_groups: "approve_new_topics_unless_trust_level"
+      email_in_allowed_groups: "email_in_min_trust"
 
     placeholder:
       discourse_connect_provider_secrets:

config/site_settings.yml

@@ -1254,6 +1254,13 @@ email:
   email_in_min_trust:
     default: 2
     enum: "TrustLevelSetting"
+    hidden: true
+  email_in_allowed_groups:
+    default: 12
+    type: group_list
+    allow_any: false
+    refresh: true
+    validator: "AtLeastOneGroupValidator"
   email_in_authserv_id:
     default: ""
   email_in_spam_header: