Changes

docs-pdf: remove v5.1 and v7.5 as they are archived  (#3029)

### **User description**
v5.1 and v7.5 PDFs are archived and will no longer trigger new PDF
builds.


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### **PR Type**
configuration changes


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### **Description**
- Removed `release-5.1` branch from postsubmits configuration in
`docs-cn-postsubmits.yaml` and `docs-postsubmits.yaml`.
- Removed `release-7.6` branch from postsubmits configuration in
`docs-cn-postsubmits.yaml` and `docs-postsubmits.yaml`.



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### **Changes walkthrough** 📝
<table><thead><tr><th></th><th align="left">Relevant
files</th></tr></thead><tbody><tr><td><strong>Configuration
changes</strong></td><td><table>
<tr>
  <td>
    <details>
<summary><strong>docs-cn-postsubmits.yaml</strong><dd><code>Update
postsubmits configuration for archived branches</code>&nbsp; &nbsp;
&nbsp; </dd></summary>
<hr>

prow-jobs/pingcap/docs/docs-cn-postsubmits.yaml

<li>Removed branch <code>release-5.1</code> from postsubmits<br> <li>
Removed branch <code>release-7.6</code> from postsubmits<br>


</details>


  </td>
<td><a
href="https://github.com/PingCAP-QE/ci/pull/3029/files#diff-a8bd1b126b4211a56553a1a30b5216df53edaba89d5a577790b6832288b489e4">+2/-2</a>&nbsp;
&nbsp; &nbsp; </td>

</tr>                    

<tr>
  <td>
    <details>
<summary><strong>docs-postsubmits.yaml</strong><dd><code>Update
postsubmits configuration for archived branches</code>&nbsp; &nbsp;
&nbsp; </dd></summary>
<hr>

prow-jobs/pingcap/docs/docs-postsubmits.yaml

<li>Removed branch <code>release-5.1</code> from postsubmits<br> <li>
Removed branch <code>release-7.6</code> from postsubmits<br>


</details>


  </td>
<td><a
href="https://github.com/PingCAP-QE/ci/pull/3029/files#diff-b1066d5a5f50efe588f78cadcc9c9441d11c1b017c00a85627e2af77af140bf7">+2/-2</a>&nbsp;
&nbsp; &nbsp; </td>

</tr>                    
</table></td></tr></tr></tbody></table>

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 (commit: 8bb627b)