Deprecate Simple Background image formatter module

[joeparsons]

Motivation

The Simple Background image formatter module that was first added to Quickstart 2 back when we were still experimenting with Layout Builder (in the very early days) is no longer being used by anything in Quickstart so we should consider deprecating it. (and eventually removing it)

• Module was added as part of Capture akslay's layout builder work #38 / Fixes #38: Initial export of akslay's layout builder work #39
• Most of what was added in Fixes #38: Initial export of akslay's layout builder work #39 was removed as part of Remove Layout Builder Customizations #280 / Fixes #280, fixes #277 Remove Layout Builder Customizations #281
• The module is not installed by default (and never has been AFAICT)

We probably should have noticed that the module was no longer being used before forking it to maintain D10 compatibility (see #2354 and #2656) 🤦🏻‍♂️.

Proposed Resolution

• Deprecate module starting with 2.8.0
  • Add status report warning if module is installed (?)
• Remove module in 2.9.0