Subscriptions API: deprecated 422 response and initialEvent wording (follow-up to #119, #121) #135

Problem description
Items from #119 and #121 that remain in device-data-volume-subscriptions.yaml in the r2.2 snapshot:

• The 422 response of POST /subscriptions still references CreateSubscriptionUnprocessableEntity422 (L228), which Commonalities r4.4 deprecates in favour of CreateSubscriptionDevice422.
• The initialEvent description triggers an event when the remaining allowance "is less than 10%" (L732), while info.description (L51) and the event schema descriptions use "equal to" the threshold.

Expected behavior

• Reference CreateSubscriptionDevice422 for the 422 response.
• Use the same threshold condition in the initialEvent description as in info.description and the event schemas.

Alternative solution
None

Additional context
Found during release review of #133. Follow-up to #119 and #121.