Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/docs/reference/setup-tools/kubeadm/generated/kubeadm_upgrade.md

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+<!-- 
+Upgrade your cluster smoothly to a newer version with this command 
+-->
+
+此命令能将集群平滑升级到新版本
+
+<!--
+### Synopsis
+-->
+
+### 概要
+
+<!--
+Upgrade your cluster smoothly to a newer version with this command
+-->
+
+此命令能将集群平滑升级到新版本
+
+```
+kubeadm upgrade [flags]
+```
+
+<!--
+### Options
+-->
+
+### 选项
+
+   <table style="width: 100%; table-layout: fixed;">
+<colgroup>
+<col span="1" style="width: 10px;" />
+<col span="1" />
+</colgroup>
+<tbody>
+
+<tr>
+<td colspan="2">-h, --help</td>
+</tr>
+<tr>
+<td></td><td style="line-height: 130%; word-wrap: break-word;">
+<!--
+<p>help for upgrade</p>
+-->
+<p>upgrade 操作的帮助命令<p>
+</td>
+</tr>
+
+</tbody>
+</table>
+
+<!--
+### Options inherited from parent commands
+-->
+
+### 继承于父命令的选项
+
+   <table style="width: 100%; table-layout: fixed;">
+<colgroup>
+<col span="1" style="width: 10px;" />
+<col span="1" />
+</colgroup>
+<tbody>
+
+<tr>
+<td colspan="2">--rootfs string</td>
+</tr>
+<tr>
+<td></td><td style="line-height: 130%; word-wrap: break-word;">
+<!--
+<p>[EXPERIMENTAL] The path to the 'real' host root filesystem.</p>
+-->
+<p>[实验] 指向 '真实' 宿主机根文件系统的路径。<p>
+</td>
+</tr>
+
+</tbody>
+</table>
+