utml.yaml: PUT /dss/v1/uss_availability/{uss_id}: old_version check is not implemented

[mickmis]

Describe the bug
According to the description of the field old_version of the PUT /dss/v1/uss_availability/{uss_id} endpoint of utm.yaml, the endpoint is supposed to support a form of compare-and-swap operation:

Version of USS's availability to change, for consistent read-modify-write operations and consistent retry behavior.

However this is not currently implemented in:

dss/pkg/scd/uss_availability_handler.go

Line 75 in 2c0ff28

func (a *Server) SetUssAvailability(ctx context.Context, req *restapi.SetUssAvailabilityRequest,

Expected behavior
The endpoint implements the behavior described by the OpenAPI specifications.