Refactor metadata usage in CI Rules entry

What does this MR do and why?

Entry::Rules is shared between the Workflow and Processable entries. Normally, we support only the if, changes, exists, when, and variables keys in workflow:rules. Yet, we accidentally allow start_in, allow_failure, and needs.

In this change, we don't aim to fix this, this will be done in Eliminate using `allowed_when` and `allow_keys` (#436473).

In this change, we aim to simplify and highlight this behavior by adding allowed_keys in the Rules usage.

Also, in validations, we start using only allowed_when and allowed_keys, we don't use the class constants anymore.

This step will help implementing workflow:rules:auto_cancel (#436467 - closed).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.