APIv2 Serializer - sla_days_remaining in FindingSerializer should have_null set to True

[jynolen]

Be informative
I use OpenAPI spec to generate me a nice client without too much effort but the generated models (or the spec in Django) might be wrong

Bug description
I use https://openapi-generator.tech/ to generate me a Client + Models from Swgger Json "/api/v2/oa3/schema/?format=json"
So far all good.

However when I try to create report for engagment I got validation issues becasue some findings (Severity = INFO) have empty SLA, so null value for sla_days_remaining

so this line (https://github.com/DefectDojo/django-DefectDojo/blob/master/dojo/api_v2/serializers.py#L1656) should be patch

sla_days_remaining = serializers.IntegerField(read_only=True) to
sla_days_remaining = serializers.IntegerField(read_only=True, allow_null=True)

And/Or give the ability to set SLA for Info findings.