docs: add deprecation notice to cron on docs/job-specification/periodic

[Antvirf]

Fixes #23391

There are quite a few different approaches to mark deprecation across the docs, I picked this as it was the most explicit parameter-level option in my opinion.

Screenshot below:

Other refs:

• The PR that added the original deprecation notice to the tool itself Add multiple times in periodic jobs #17858
• Changelog entry mentioning that PR for Nomad 1.6.2 https://github.com/hashicorp/nomad/blob/main/CHANGELOG.md#162-september-13-2023

[hashicorp-cla-app]

All committers have signed the CLA.

[tgross]

Thanks for picking this up, @Antvirf. Changing the parameter to (string: <optional>) might be a little confusing though... I get where you're going with that, but it's required if crons is unset. Let's change it just (string) and change the note at the end of crons to:

Either cron or crons must be set, but not both.

And then add the same note to the end of cron too.

[Antvirf]

@tgross done 🫡

[tgross]

LGTM! Thanks @Antvirf!

Preview link: https://nomad-7e30x9qv1-hashicorp.vercel.app/nomad/docs/job-specification/periodic#periodic-parameters