Remove mysql2 gem version requirement "< 0.6.0"

Suggested at https://github.com/rails/rails/pull/33876#issuecomment-421176221

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -3,7 +3,7 @@
 require "active_record/connection_adapters/abstract_mysql_adapter"
 require "active_record/connection_adapters/mysql/database_statements"
 
-gem "mysql2", ">= 0.4.4", "< 0.6.0"
+gem "mysql2", ">= 0.4.4"
 require "mysql2"
 
 module ActiveRecord

railties/lib/rails/generators/app_base.rb

@@ -299,7 +299,7 @@ def rails_version_specifier(gem_version = Rails.gem_version)
       def gem_for_database
         # %w( mysql postgresql sqlite3 oracle frontbase ibm_db sqlserver jdbcmysql jdbcsqlite3 jdbcpostgresql )
         case options[:database]
-        when "mysql"          then ["mysql2", [">= 0.4.4", "< 0.6.0"]]
+        when "mysql"          then ["mysql2", [">= 0.4.4"]]
         when "postgresql"     then ["pg", [">= 0.18", "< 2.0"]]
         when "oracle"         then ["activerecord-oracle_enhanced-adapter", nil]
         when "frontbase"      then ["ruby-frontbase", nil]

railties/test/generators/app_generator_test.rb

@@ -492,7 +492,7 @@ def test_config_mysql_database
     if defined?(JRUBY_VERSION)
       assert_gem "activerecord-jdbcmysql-adapter"
     else
-      assert_gem "mysql2", "'>= 0.4.4', '< 0.6.0'"
+      assert_gem "mysql2", "'>= 0.4.4'"
     end
   end