DEV: Deprecate defunct User#flag_level column (#24134)

The User#flag_level column has not been in use for a very long time. The "new" reviewable system dynamically calculates flag scores based on past performance of the user.

This PR removes flag_level from the admin user serializer (since it isn't displayed anywhere in admin user lists) and marks the column as deprecated and targeted for removal in the next minor version.

app/models/user.rb

@@ -6,6 +6,7 @@ class User < ActiveRecord::Base
   include HasCustomFields
   include SecondFactorManager
   include HasDestroyedWebHook
+  include HasDeprecatedColumns
 
   DEFAULT_FEATURED_BADGE_COUNT = 3
 
@@ -13,6 +14,8 @@ class User < ActiveRecord::Base
   TARGET_PASSWORD_ALGORITHM =
     "$pbkdf2-#{Rails.configuration.pbkdf2_algorithm}$i=#{Rails.configuration.pbkdf2_iterations},l=32$"
 
+  deprecate_column :flag_level, drop_from: "3.2"
+
   # not deleted on user delete
   has_many :posts
   has_many :topics

app/serializers/admin_user_list_serializer.rb

@@ -14,7 +14,6 @@ class AdminUserListSerializer < BasicUserSerializer
              :created_at_age,
              :trust_level,
              :manual_locked_trust_level,
-             :flag_level,
              :username,
              :title,
              :avatar_template,

spec/requests/api/schemas/json/admin_user_list_response.json

@@ -65,9 +65,6 @@
           "null"
         ]
       },
-      "flag_level": {
-        "type": "integer"
-      },
       "title": {
         "type": [
           "string",
@@ -109,7 +106,6 @@
       "created_at_age",
       "trust_level",
       "manual_locked_trust_level",
-      "flag_level",
       "title",
       "time_read",
       "staged",

spec/requests/api/schemas/json/admin_user_response.json

@@ -46,9 +46,6 @@
     "manual_locked_trust_level": {
       "type": ["string", "null"]
     },
-    "flag_level": {
-      "type": "integer"
-    },
     "title": {
       "type": ["string", "null"]
     },
@@ -509,7 +506,6 @@
     "created_at_age",
     "trust_level",
     "manual_locked_trust_level",
-    "flag_level",
     "title",
     "time_read",
     "staged",