Spring Framework 6.0 recipe with migration for Spring Assert

[pativa]

What's changed?

Added a base recipe for migrating to Spring Framework 6.
This is not a complete migration, but can serve as a place to put Spring-related migrations necessary (as opposed to changes related to only Spring Boot).

What's your motivation?

#521

Anything in particular you'd like reviewers to focus on?

I copied the format from the earlier UpgradeSpringFramework_5_3.

Any additional context

I have tested the changes under MigrateSpringAssert in our internal migration and it seems to work fine.
I have taken the literal text added from the Spring class and reformatted them a bit.

Ideally, the message contain variable name but that would be a harder change to write automatically.
Usage of this is probably not too common anyway as the functions have been deprecated for a long time.

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[timtebeek]

Great to see; thanks a lot!