v0.13.0

• Null-safety support: both in the library and the generated code.
• Remove deprecated arguments offset, limit, withEqual from Query methods.
• More performance optimizations, mostly in our FlatBuffers fork.
• Fix FlatBuffers builder growing.
• Update to objectbox-c v0.13.0
• Update to objectbox-android v2.9.1
• Increase minimum SDK versions: Flutter v2.0 & Dart v2.12.