Fix boolean columns triggering MySQL deprecation

Previously, when creating a boolean column for a MySQL database

```ruby
class AddSoldToProduct < ActiveRecord::Migration[8.0]
  def change
    add_column :products, :sold, :boolean
  end
end
```

Active Record would create a query like

```SQL
ALTER TABLE `products` ADD `sold` tinyint(1)
```

which would trigger a deprecation warning in MySQL 8.0.17+

```
1681: Integer display width is deprecated and will be removed in a future release.
```

Additionally, starting in MySQL 8.0.19, integer display widths are no
longer included in the output of `SHOW CREATE TABLES` with the exception
of `tinyint(1)` because

> MySQL Connectors make the assumption that TINYINT(1) columns
> originated as BOOLEAN columns; this exception enables them to continue
> to make that assumption.

This commit changes Active Record to use the `boolean` alias for MySQL
(which still results in `tinyint(1)` columns) but does not emit the
deprecation warning for using an integer display width.

```SQL
ALTER TABLE `products` ADD `sold` boolean
```

Author: skipkayhil

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -42,7 +42,7 @@ class AbstractMysqlAdapter < AbstractAdapter
         date:        { name: "date" },
         binary:      { name: "blob" },
         blob:        { name: "blob" },
-        boolean:     { name: "tinyint", limit: 1 },
+        boolean:     { name: "boolean" },
         json:        { name: "json" },
       }