Remove config default value

[jberger]

Having a stash default "config" which pointed to the app's config was a backcompat measure ages ago. Most users use the config helper now. It is awkward that the default is only loaded by the config loader plugins and makes for unexpected action-at-a-distance since that default does not exist if you load configuration in other ways. It should be removed in favor of the config helper and/or app accessor.

Mitigations for affected users:
Users are encouraged to switch to using config in templates rather than $config. Projects wishing to keep the existing behavior may implement the default in the project themselves by adding $app->defaults(config => $app->config); to the application startup after loading their configuration.

[kraih]

Deprecation is now in effect. baf97a0...917d61d

[kraih]

This is a test comment to see if linking to a discussion like #1625 works.

[kraih]

Done, as part of the 9.0 branch.