Update method signatures and class definitions to use full 8.0 typing

[JordanRL]

Now that the minimum version of PHP for the project has been set at 8.0, it makes sense to update all of the interfaces, classes, and methods to utilize the proper union types.

The same process would need to be done for each of the modules individually (which would also increment their minor version since public signatures change).

• [x] #97
• [x] #98
• [x] #99
• [x] #100
• [x] #101