Update migration examples to showcase database-agnostic raw SQL execution

[wpank84]

Updated the raw SQL shown in the migration code examples of sections 3.10, 3.11, and 3.12.

The previous example would error out in SQLite due to SQLite not supporting adding constraints in the ALTER TABLE command.

Motivation / Background

Fixes #47941

Detail

This Pull Request changes the code examples of active_record_migrations.md in sections 3.10, 3.11, and 3.12.

[yahonda]

Thanks for opening this pull request.

This section aims to explain how to make migrations reversible when the existing migration method is not reversible. However, these SQL statements in the pull request migration are already reversible using the `add_index' method.

rails/guides/source/active_record_migrations.md

Lines 755 to 768 in cc359c0

	### Using the `up`/`down` Methods
	You can also use the old style of migration using `up` and `down` methods
	instead of the `change` method.
	The `up` method should describe the transformation you'd like to make to your
	schema, and the `down` method of your migration should revert the
	transformations done by the `up` method. In other words, the database schema
	should be unchanged if you do an `up` followed by a `down`.
	For example, if you create a table in the `up` method, you should drop it in the
	`down` method. It is wise to perform the transformations in precisely the
	reverse order they were made in the `up` method. The example in the `reversible`
	section is equivalent to:

For example, the current example has been updated from foreign key to check constraint if Rails migration supports add_foreign_key' and remove_foreign_key'. Refer to #15606

Would you provide some SQL statements which are not supported by Rails change method and work for all supported databases like PostgreSQL, MySQL and SQLite3?

[wpank84]

@yahonda Appreciate the feedback! I went ahead and made a new commit using CREATE VIEW for the raw SQL which, to my knowledge, should satisfy those conditions.

[yahonda]

It looks good. Would you squash your commits into a single commit?

[wpank84]

Squashed the commit to a single commit and should be good to go