Add content to upgrade guide for v6 major bump

[jchen293]

Description

Add content for the V6 major bump in the upgrade guide
Move the ExternalApiError from the general to API folder

Testing

N/A

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[Justintime50]

I'd argue we need the following items in the upgrade guide too:

• Changes the type result of Event from EasyPostResource to Map<String, Object>
• Changes the type of Insurance Amount from Float to String
• Removes all the setters of each object (we say setters are available via lombok; however, I don't think we added the decorator meaning users can't access them. In the off chance someone was previously, we need to call it out that they can't any longer)
• Rename some getters
  • Pickup class: getPickoutRates() -> getPickupRates()
  • PaymentMethod class: getPrimaryPaymentMethodObject() -> getPrimaryPaymentMethod()
  • PaymentMethod class: getSecondaryPaymentMethodObject() -> getSecondaryPaymentMethod()

The point of the upgrade guide is to callout anything that will require a user to change their code and provide guidance and examples on how to do it to ease the process of moving from one major version to another. We don't need to call out additions here (the CHANGELOG can handle that), but the upgrade guide should include anything substantial that will require an engineer to make a tweak in a codebase downstream from us.

[Justintime50]

Various small nits to fix grammar/spelling