Refactor metadata usage in CI Rules entry

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
#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.