Fix kubectl server-side dry-run flag

[jenting]

Why is this PR needed?

The flag of server-side dry-run is different at k8s client < 1.18.0 and k8s client >= 1.18.0.

What does this PR do?

Check the kubectl client version and return the corresponding server-side dry-run flag.
If k8s client version < v1.18.0, the server-side dry-run flag is --server-dry-run.
If k8s client version >= v1.18.0, the server-side dry-run flag is --dry-run=server.

Anything else a reviewer needs to know?

N/A

Info for QA

This is info for QA so that they can validate this. This is mandatory if this PR fixes a bug.
If this is a new feature, a good description in "What does this PR do" may be enough.

Related info

N/A

Status BEFORE applying the patch

1. The local kubectl CLI version < 1.18.0
2. Deploy a cluster by previous skuba CLI.
3. Use the new skuba CLI to perform cluster upgrade and add-on upgrade.
4. The error message display on the console.

Status AFTER applying the patch

1. The local kubectl CLI version < 1.18.0
2. Deploy a cluster by previous skuba CLI.
3. Use the new skuba CLI to perform cluster upgrade and add-on upgrade.
4. The add-on upgrade success.

AND

1. The local kubectl CLI version >= 1.18.0
2. Deploy a cluster by previous skuba CLI.
3. Use the new skuba CLI to perform cluster upgrade and add-on upgrade.
4. The add-on upgrade success.

Docs

N/A

Merge restrictions

(Please do not edit this)

We are in v4-maintenance phase, so we will restrict what can be merged to prevent unexpected surprises:

What can be merged (merge criteria):
    2 approvals:
        1 developer: code is fine
        1 QA: QA is fine
    there is a PR for updating documentation (or a statement that this is not needed)

[mjura]

LGTM