Improve branching and release models

[briantopping]

How to categorize this issue?
/kind enhancement
/platform vsphere

What would you like to be added
Maintain Gardener-standard branching model to gardener-extension-provider-vsphere and machine-controller-manager-provider-vsphere to support a matrix of versions between Gardener, Kubernetes and vSphere.

Why is this needed
In conjunction with #204, the community is served by users being able to discover releases on several different version axes:

• For a given Gardener release level, what versions of vSphere are supported
• What versions of Gardener (and Kubernetes) are possible for a given vSphere foundation?

While simulating vSphere in vcsim, the extension should be tested with various VI SDK version levels. vmware/govmomi#1214 seems to imply that this is not available, need to confirm. In any event, this extension should be tested against specific VI SDK levels for a given release.

Over time, newer extensions will take advantage of newer VI SDK features, making the extension backwards-incompatible. In that case, a maintainable release branch should be cut. The older release branches will allow minor dependency upgrades and point fixes to be applied to older versions, supporting deployments that cannot upgrade to newer versions for organizational or site reasons. This will create parallel release tracks. The trigger for these parallel release tracks would be gathered either with ad-hoc knowledge about the community or via issues that the community raises. Opt-in telemetry could also be used to report this information if it existed.