Mark plugin-/engine-Tags as deprecated

# Bug Report
On a conversation on slack it turned out, that plugin-Tags and engine-Tags are deprecated to use in config.xml. It should be clear for the user, that these tags are deprecated and they should be only maintained by the package.json-File.

## Problem
There is no hint, that these tags are deprecated. The documentation don't say anything, nor the cli.
Some old users could still use the plugin- and engine-Tags in the config.xml and could be wondered what is the right way.

### What is expected to happen?
There should be somewhere a hint, that these tags are deprecated.

### What does actually happen?
There is nowhere a hint, that these tags are deprecated.

## Checklist

- [ ] I searched for existing GitHub issues
- [ ] I updated all Cordova tooling to most recent version
- [ ] I included all the necessary information above

Start by locating the config.xml handling for plugin-Tags and engine-Tags, then check where the related documentation and CLI messages are defined. Done means users are clearly told these tags are deprecated and that package.json is the maintained source.

Written by the indexing model from the issue text.