API v2 note get

[c8y3]

Implement endpoint GET /api/v2/cases/{case_identifier}/notes/{identifier} to get a note.

• [x] return 200 in case of success
• [x] if the case is not found, return 404
• [x] if the note is not found, return 404
• [x] create method in the business layer
• [x] check returned object is uniform with other endpoints
• [x] deprecate GET /case/notes/{note_id}
• [x] openapi: document endpoint
• [x] openapi: deprecate POST /case/notes/{note_id}

[c8y3]

• [x] introduction of iso_date.yaml in documentation
• [x] openapi: add links for deprecated endpoints