chore: migrate jetbrains annotations to jspecify

[powercasgamer]

closes #982

• [x] api
• [x] nbt
• [x] key
• [x] configurate3
• [x] configurate4
• [x] text-logger-slf4j
• [x] text-minimessage
• [x] text-serializer-ansi
• [x] text-serializer-gson
• [x] text-serializer-gson-legacy-impl
• [x] text-serializer-json
• [x] text-serializer-json-legacy-impl
• [x] text-serializer-legacy
• [x] text-serializer-plain

[kezz]

This should be using package/class level @NullMarked annotations