Clarify version of semantic versioning of maintenance_info.version

[nmaslarski]

What is the problem this PR solves?

So that when semver 2.1 or 3.0 gets released, it would be clear which version should Brokers and Platforms implement, and will allow OSBAPI group to update the referenced version in one of the next OSBAPI releases when it makes sense to do so. It gives more control over what that version field even means.

Checklist:

• [x] The swagger.yaml doc has been updated with any required changes (not applicable)
• [x] The openapi.yaml doc has been updated with any required changes (not applicable)

[cfdreddbot]

:white_check_mark: Hey nmaslarski! The commit authors and yourself have already signed the CLA.