Documentation: dev-tools: properly format ToC headingss

"class:: toc-title" was a workaround for older Sphinx versions that are
no longer supported.

The canonical way to add a heading to the ToC is to use :caption:.
Do that.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: gaochao <gaochao49@huawei.com>
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Wu XiangCheng <bobwxc@email.cn>
Cc: Fangrui Song <maskray@google.com>
Cc: Wan Jiabing <wanjiabing@vivo.com>
Cc: Alex Shi <alexs@kernel.org>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-6-vegard.nossum@oracle.com

Documentation/dev-tools/index.rst

@@ -10,11 +10,8 @@ whole; patches welcome!
 A brief overview of testing-specific tools can be found in
 Documentation/dev-tools/testing-overview.rst
 
-.. class:: toc-title
-
-	   Table of contents
-
 .. toctree::
+   :caption: Table of contents
    :maxdepth: 2
 
    testing-overview

Documentation/translations/zh_CN/dev-tools/index.rst

@@ -14,11 +14,8 @@
 有关测试专用工具的简要概述，参见
 Documentation/translations/zh_CN/dev-tools/testing-overview.rst
 
-.. class:: toc-title
-
-	   目录
-
 .. toctree::
+   :caption: 目录
    :maxdepth: 2
 
    testing-overview