Notify community of v3.0.0 schema version breaking changes

[annakrystalli]

Given v3.0.0 contains breaking changes that will affect many packages in the hubverse it is a good idea to communicate the changes to the community.

Steps involved:

• [x] Prepare draft announcement
• [x] Create Announcement on org dicsussion board
• [x] Send Email announcement

[annakrystalli]

Draft announcement can be found here

[annakrystalli]

Announcement can be found here https://github.com/orgs/Infectious-Disease-Modeling-Hubs/discussions/13 and email notifcation sent out by @mmkerr on same day