[TASK] Clarify the reason for k8s.io replace directives in go.mod files

[ejweber]

What's the task? Please describe

Add clarifying comments to any Longhorn go.mod files that import k8s.io/kubernetes (and thus, include replace directives).

Describe the sub-tasks

• longhorn-manager: master - Add replace directive clarifying comment longhorn-manager#2766
• longhorn-manager: v1.6.x - Add replace directive clarifying comment (backport #2766) longhorn-manager#2768
• longhorn-manager: v1.5.x - Add replace directive clarifying comment (backport #2766) longhorn-manager#2769
• longhorn-share-manager: master - Add replace directive clarifying comment longhorn-share-manager#205
• longhorn-share-manager: v1.6.x - Add replace directive clarifying comment (backport #205) longhorn-share-manager#206
• longhorn-share-manager: v1.5.x - Add replace directive clarifying comment (backport #205) longhorn-share-manager#207

[ejweber]

Closing this directly since the comments have all been added.