CI: Update OS version in Read the Docs configuration (#5392)

https://about.readthedocs.com/blog/2026/03/ubuntu-20-04-deprecated/

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Updated documentation build environment to use a newer OS image,
maintaining compatibility with existing Python toolchain and build
configuration.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@ustc.edu.cn>

Author: njzjz

.readthedocs.yml

@@ -1,6 +1,6 @@
 version: 2
 build:
-  os: ubuntu-20.04
+  os: ubuntu-26.04
   tools:
     python: "3.11"
   jobs: