update docs to reflect deprecation of dot-based config navigation

[kgeis]

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Issue description

I am preparing to migrate from Grails 4 to 5, and the release notes talk about deprecation of the dot-based config navigation. I want to see the recommended practice for setting values on my Spring beans, so I look in Grails 5.1.2 Guide: Grails and Spring. The examples there use the deprecated navigation and should be updated.

[jeffscottbrown]

@kgeis Thank you for the feedback. It is good information. We will investigate updating the docs.

In the meantime, some more information related to retrieving config values is available at grails.org/blog/2017-01-20-3.html and grails.org/blog/index.html.

Thank you!