Newsletter: Make it more clear that Newsletter doesn't work with old posts (or allow it to send old posts)

### Impacted plugin

Jetpack

### What

Jetpack Newsletter only sends notifications for new posts, yet, the options are active for old posts which gives the impression can Newsletter can be used with them. For these old posts, could we grey out the Newsletter options, and have some verbiage to explain why.

We could also share a workaround or link to a doc for that. (A workaround in mind is to create a new post with a link to the old post). Also, maybe also show the Newsletter options under the Post settings.

### How

Brainstorming an idea:

1. Detect if the post is old, this can be a post that is:
- already published
- was published but switched back to draft, then published again
2. If the post is considered old, then disable the Newsletter settings in the editor (at the pre-publish checks stage)

If the Newsletter options could also show under the Post settings, it could help make it more obvious that Newsletter can't be used for old posts. Right now, the Post settings do not list Newsletter.


---
_Update comment_

Internal discussion: p1717226457719859-slack-C052XEUUBL4

Ideas suggested by @lezama:
- Make it more clear than Newsletter won't work by adding messaging to the Newsletter panel if the post was already published before.
- Allow user to send the email notification by adding tooling to send the newsletter if it was never sent before, like add a button to send the newsletter at a later time if the original publishing decision was to “Post Only.”

---

User report: https://wordpress.org/support/topic/jetpack-newsletter-old-post-dont-send/

Start with the Jetpack Newsletter panel in the editor and its pre-publish checks, as described in the issue. Determine how previously published or republished posts are identified, then clarify whether the desired result is disabled settings with explanatory text, a way to send the newsletter later, or both; done should make the limitation clear and support the agreed behavior.

Written by the indexing model from the issue text.