`soa_serial_dirty` is a class level, not an object level attribute

[peteeckel]

Versions
NetBox Version: 4.0.7
NetBox DNS Version: 1.0.4
Python Version: 3.11.5

Describe the bug
Not really a bug affecting functionality.

While working on optimizing the performance of zone and record operations, an internal flag field soa_serial_dirty was introduced to the Zone model. That field is used for tracking whether changes to a zone or its records make it necessary to calculate a new SOA Serial value.

That field is unfortunately defined on class level, not on object level. The result is that if multiple zones are changed at the same time, too many new calculations of the SOA Serial might take place.