Add "Upgrading" section to vSphere Integration page

Foreword

From the vSphere docs:

Kubernetes will deprecate the in-tree vSphere volume plug-in, and it will be removed in the future Kubernetes releases. Volumes provisioned using the vSphere in-tree plug-in will not get additional new features supported by the vSphere Container Storage Plug-in.
Kubernetes provides a seamless procedure to help migrate in-tree vSphere volumes to a vSphere Container Storage Plug-in. After you migrate the in-tree vSphere volumes vSphere Container Storage Plug-in, all subsequent operations on migrated volumes are performed by the vSphere Container Storage Plug-in. The migrated vSphere volume will not get additional capabilities supported by vSphere Container Storage Plug-in.

Add a section that guides users through the vSphere integration upgrade process to 1.26 and helps them migrate old "in-tree" volumes to the new "out-of-tree" provider.

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