[EventGrid] Prepare for Release

[ellismg]

• Remove the "FarmBeats" system events (i.e. AgriFoodFarming). The
  service team is unwilling to GA the shape of the events as this
  time.

• Set release date in CHANGELOG.md

[ellismg]

After talking with the service team, we've decided to remove the FarmBeats system events for now as there is some concern about incompatible changes been made between now and when the service itself GA. They will return when we have higher confidence the event schemas are stable.

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