C2MS12 — Fix Use of 'Severe' in Diagrams (Should be 'Critical')

Late in C2MS 1.1, we aligned SDTF error levels across the specs. 1.1 initially used 'Severe' for the highest level of concern. Later, we changed this to 'Critical' as this was better alignment with XTCE. Although the document text was all changed, we had three model diagrams that referred to 'Severe' in notes: Log Message, Heartbeat Message and Device Message. The notes in these diagrams all need to be fixed to use 'Critical' instead of 'Severe'.

Note that Log Message is already being fixed as part of the proposed resolution for C2MS12-43. Therefore, this issue only addresses Heartbeat and Device Messages.