ActiveRecord adapter: Fix migration error for MySQL

[bkeepers]

• Run ActiveRecord adapter tests against mysql, postgres, and sqlite
• Fix index for mysql
• Configure mysql/postgres to run on GitHub Actions
• Verify that index length does not adversely effect postgres or other databases
• Merge Add rails generate flipper:update to generate migrations for updates #787 and update migration

Fixes #789

[bkeepers]

[x] Verify that index length does not adversely effect postgres or other databases

@jnunemaker I tested against Flipper Cloud that the length: {value: 255} argument is ignored by postgres by running the migration below.

class ChangeFlipperGatesIndex < ActiveRecord::Migration[7.1]
  disable_ddl_transaction!

  def change
    remove_index :flipper_gates, [:feature_key, :key, :value], unique: true
    add_index :flipper_gates, [:feature_key, :key, :value], unique: true, length: {value: 255}, algorithm: :concurrently
  end
end

And there is no changes to the schema

$ git diff
diff --git a/db/structure.sql b/db/structure.sql
index 70c58e57..4782732b 100644
--- a/db/structure.sql
+++ b/db/structure.sql
@@ -2771,6 +2771,7 @@ ALTER TABLE ONLY public.features
 SET search_path TO "$user", public;

 INSERT INTO "schema_migrations" (version) VALUES
+('20231211220438'),
 ('20231126182713'),
 ('20231126182525'),
 ('20231126182250'),