Migrate floor_generator's dependencies to null-safe versions

[vitusortner]

We have recently migrated the codebase to support null-safety (#462). We're now generating null-safe code but floor_generator's internal dependencies aren't exclusively null-safe.

Migrate the dependencies to their null-safe versions once available and make sure to not run tests with --no-sound-null-safety anymore (CI).

• build
• build_config
• build_test
• code_builder
• dart_style
• source_gen

Additionally, make sure to get rid of the mockito (5.0.2) dependency override, which was introduced to make floor work with source_gen 1.0.0. The tracking issue for mockito can be found here: dart-lang/mockito#373.

[rickydj]

Please, fix this as soon as possible, i cant build entities because i have problems with this as it cant be null safety, the system said: "Query method parameters have to be non-nullable. Define id as non-nullable."

[vitusortner]

Please, fix this as soon as possible, i cant build entities because i have problems with this as it cant be null safety, the system said: "Query method parameters have to be non-nullable. Define id as non-nullable."

We're tracking this problem in a separate issue #507.