Add a CI step to validate that the example migrations are valid
according to the JSON schema.

This would have caught the problem where the JSON schema was not fully
updated in #293 and had to be
updated in a follow-up PR #300
despite #293 including an example migration.

Use [ajv-cli](https://ajv.js.org/packages/ajv-cli.html) to perform
schema validation.

Here is an [example
run](https://github.com/xataio/pgroll/actions/runs/8113707179/job/22177613982)
where the check fails.