Related Posts: Add warning / JITM on template page if switching to FSE theme

### Impacted plugin

Jetpack

### What

Currently, Related Posts don't show by default on FSE themes (since Jetpack 11.1), unless added using the block. For anyone moving from a non-FSE theme to an FSE theme, this change will be unexpected.

On FSE-themed sites with a lot of pages / posts eligible to show Related Posts, the Related Posts block can be added to the relevant template (for example the single post template) in order for the Related Posts to show at the bottom.

It was suggested here - pcjTuq-hx-p2#comment-422 - that a warning or JITM be added to the theme template admin page on FSE-themed sites whenever Related Posts are enabled, to recommend using the block. Also when switching from classic to FSE theme, we can recommend using the block. We can also update the Related Posts settings card to be a little clearer about available options (at `wp-admin/admin.php?page=jetpack#/traffic` ). This would help avoid worry over the loss of a potentially major feature.

Related internal communication: p1667277686472659-slack-C043B54G6V7

Related issues:
* Adding the title to the block - https://github.com/Automattic/jetpack/issues/27108
* The PR where Related Posts on FSE themes stopped working: https://github.com/Automattic/jetpack/pull/24622

Start at the Jetpack Traffic settings entry point, wp-admin/admin.php?page=jetpack#/traffic, and review how FSE theme templates and the Related Posts block are presented. Done would include an agreed warning or JITM for enabled Related Posts on FSE templates or during a theme switch, plus clearer Related Posts settings; the issue does not identify files or tests.

Written by the indexing model from the issue text.