Drop ruby 2.6 support

We just ran into something where it was inconvenient to keep; ruby 2.6 is actually already EOL; and we try NOT to drop ruby's very often between major releases, so whatever we have going into 2.0 we're going to be stuck with for a while.

Let's drop 2.6 now.

.github/workflows/ci.yml

@@ -29,9 +29,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - gemfile: rails_6_0
-            ruby: 2.6
-
           - gemfile: rails_6_1
             ruby: 2.7
 

CHANGELOG.md

@@ -19,7 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * We now create Rails Attribute cover for all attr_json attributes, and we do a better job of keeping the Rails attribute values sync'd with attr_json values.   https://github.com/jrochkind/attr_json/pull/117, https://github.com/jrochkind/attr_json/pull/158, and https://github.com/jrochkind/attr_json/pull/163
 
-* Drop Rails earlier than 6.0 and ruby earlier than 2.6. https://github.com/jrochkind/attr_json/pull/155
+* Drop support for Rails earlier than 6.0 and ruby earlier than 2.7. https://github.com/jrochkind/attr_json/pull/155 https://github.com/jrochkind/attr_json/pull/174
 
 * Array types now default to an empty array. If you'd like to turn that off, you can use the somewhat odd `default: AttrJson::AttributeDefinition::NO_DEFAULT_PROVIDED` on attribute definiton. Thanks @g13ydson for suggestion. https://github.com/jrochkind/attr_json/pull/161
 

README.md

@@ -3,7 +3,7 @@
 [![CI Status](https://github.com/jrochkind/attr_json/workflows/CI%20on%20Future%20Rails%20Versions/badge.svg?branch=master)](https://github.com/jrochkind/attr_json/actions?query=workflow%3A%22CI+on+Future+Rails+Versions%22+branch%3Amaster)
 [![Gem Version](https://badge.fury.io/rb/attr_json.svg)](https://badge.fury.io/rb/attr_json)
 
-ActiveRecord attributes stored serialized in a json column, super smooth. For Rails 6.0.x through 7.0.x. Ruby 2.6+.
+ActiveRecord attributes stored serialized in a json column, super smooth. For Rails 6.0.x through 7.0.x. Ruby 2.7+.
 
 Typed and cast like Active Record. Supporting [nested models](#nested), [dirty tracking](#ar_attributes), some [querying](#querying) (with postgres [jsonb](https://www.postgresql.org/docs/9.5/static/datatype-json.html) contains), and [working smoothy with form builders](#forms).