Add initializer to warn on unsupported PG version

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/224683

config/initializers/1_postgresql_only.rb

@@ -2,3 +2,27 @@
 
 raise "PostgreSQL is the only supported database from GitLab 12.1" unless
   Gitlab::Database.postgresql?
+
+begin
+  puts ERB.new(Rainbow.new.wrap(<<~EOS).red).result unless Gitlab::Database.postgresql_minimum_supported_version? # rubocop:disable Rails/Output
+
+              ██     ██  █████  ██████  ███    ██ ██ ███    ██  ██████ 
+              ██     ██ ██   ██ ██   ██ ████   ██ ██ ████   ██ ██      
+              ██  █  ██ ███████ ██████  ██ ██  ██ ██ ██ ██  ██ ██   ███ 
+              ██ ███ ██ ██   ██ ██   ██ ██  ██ ██ ██ ██  ██ ██ ██    ██ 
+               ███ ███  ██   ██ ██   ██ ██   ████ ██ ██   ████  ██████  
+
+    ******************************************************************************
+      You are using PostgreSQL <%= Gitlab::Database.version %>, but PostgreSQL >= <%= Gitlab::Database::MINIMUM_POSTGRES_VERSION %>
+      is required for this version of GitLab.
+      <% if Rails.env.development? || Rails.env.test? %>
+      If using gitlab-development-kit, please find the relevant steps here:
+        https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/master/doc/howto/postgresql.md#upgrade-postgresql
+      <% end %>
+      Please upgrade your environment to a supported PostgreSQL version, see
+      https://docs.gitlab.com/ee/install/requirements.html#database for details.
+    ******************************************************************************
+  EOS
+rescue ActiveRecord::ActiveRecordError, PG::Error
+  # ignore - happens when Rake tasks yet have to create a database, e.g. for testing
+end

lib/gitlab/database.rb

@@ -4,6 +4,8 @@ module Gitlab
   module Database
     include Gitlab::Metrics::Methods
 
+    MINIMUM_POSTGRES_VERSION = 11
+
     # https://www.postgresql.org/docs/9.2/static/datatype-numeric.html
     MAX_INT_VALUE = 2147483647
 
@@ -102,7 +104,7 @@ module Gitlab
     end
 
     def self.postgresql_minimum_supported_version?
-      version.to_f >= 11
+      version.to_f >= MINIMUM_POSTGRES_VERSION
     end
 
     def self.upsert_supported?