Added note to `webhooks.api_version` column
Added note to `webhooks.api_version` column

refs https://github.com/TryGhost/Team/issues/513
refs https://github.com/TryGhost/Team/issues/477

- We have skipped work to improve the 'defaultTo' value when working on Ghost 4.0 release, so adding this comment while context loaded
-  defaultTo should not be set to anything as it leads to more maintenance work during major version bump
- having validation might make sense but could lead to similar maintenance work unless it's linked to some global nosion of "supported API versions" used everywhere

e10/Ghost,bitjson/Ghost,TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost,ErisDS/Ghost,janvt/Ghost,bitjson/Ghost,ErisDS/Ghost,e10/Ghost,ErisDS/Ghost,janvt/Ghost