Stable C++ support in Gradle

A bucket to collect the work to be done so that we can promote the C++ support from incubating to stable:

- Announcements
- Blog posts
- User guide updates
- Change the samples to use the current model plugins
- Add `gradle init` templates
- Update the guides
- Deprecate and later remove all of the software model

Features:

- Review source layout convention and output file locations.
- Don't bake absolute paths to libraries into executables.
- Can use a dependency on an application to get a usable executable.
- Can debug an application.
- Can declare target C++ source language level.
- Customize C/C++ library name, executable name.
- Build debug variant with `-Og` when using GCC
- Merge modules for release variant on MacOS using `ld -r`

No files, tests, or entry points are named. Start by breaking the checklist into separate work items and identifying the relevant Gradle native support areas for each one. Done means the listed stability features, documentation, samples, templates, and software-model deprecation work have been completed and C++ support can be promoted from incubating to stable.

Written by the indexing model from the issue text.