Start Up Performance - Explore disabling allow-bean-definition-overriding true by default

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Feature description

Start Up Time Performance can be severely impacted by the following code:

https://github.com/grails/grails-core/blob/1631e8640f4120c00c8c9241b67fb62d698a86ee/grails-core/src/main/groovy/grails/boot/GrailsApp.groovy#L104-L107

I think best practice would be to disable this by default as it hides a lot of duplicate initialization happening behind the scenes with Spring Boot.

For instance, in certain situations I have found when setAllowBeanDefinitionOverriding(true) is on, Spring Boot AutoConfigurations are happening that perform various configurations that are later not even used and completely overridden by Grails. The more dependencies that Spring Boot detects in the class path, the greater this penalty can become.

We should explore disabling this by default as it can always be turned on independently via config if required by the end user:

spring.main.allow-bean-definition-overriding=true

or

spring:
  main:
    allow-bean-definition-overriding: true

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allow-circular-references is also something that should be considered off by default due to it promotes bad design by resulting in a tightly coupled application.