Behavior of `spec.version` when not specified: Discussion

[tmshort]

What is the behavior of spec.version when not specified? There are multiple possibilities, some of which are listed here:

1. Not valid; spec.version must be specified.
2. spec.version is optional, when not specified, nothing happens.
3. A non-specified spec.version means "latest", such that it can be used to upgrade to the latest version of an operator when it becomes available.
4. A non-specified spec.version means "any", such that when initially created, it will refer to any available version (for example, "latest"). On subsequent updates, a non-specified spec.version will match the currently installed version, so no updates occur.