[PP-1] Remove the `package_manager.settings:executables` config structure

Problem/Motivation

In #3540215: Remove the ability to configure the path to Composer, we removed support for the package_manager.settings:executables config structure. But we did not remove the config values themselves, because we don't want to break sites relying on them.

As detailed in #3540215-20: Remove the ability to configure the path to Composer, the idea is that we will remove them as part of the upgrade path to Drupal 12. In this issue, let's create that upgrade path. At the moment, I'm filing this against 11.x because 12.x isn't branched yet.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet