Prefer typed fields instead of untyped "def" fields when injecting a service (or bean/component/etc)

[gsartori]

Given that:

• Groovy suggests to not use def for class fields definition (https://groovy-lang.org/semantics.html#_variables_vs_fields_in_type_inference)
• In Grails docs prcedence is given to def when declaring injected class fields (https://docs.grails.org/6.2.3/guide/single.html#dependencyInjectionServices)
• Actually free IDE support for Groovy is available, free IDE support for Grails is not. This means autocompletion is available for free only if we define typed class fields
  -- Intellij IDEA https://plugins.jetbrains.com/plugin/20393-groovy
  -- VS Code https://marketplace.visualstudio.com/items?itemName=marlon407.code-groovy
  -- Eclipse https://marketplace.eclipse.org/content/groovy-development-tools

I propose we give precedence to typed fileds in the documentation so that Grails can be "accepted" better in more contexts promoting the feeling of being less "exclusive".

[matrei]

Yes 👍

[gsartori]

Addressed here: #13994 (comment)