docs: mark v2.x as stale in README.md

Author: yoshinorin

README.md

@@ -9,7 +9,7 @@
 || Version | CI/CD | Coverage |
 |---|---|---|---|
 | `v3.x` |![](https://img.shields.io/github/v/release/yoshinorin/qualtet?sort=semver&style=flat&label=Release)|[![CI](https://github.com/yoshinorin/qualtet/actions/workflows/ci.yml/badge.svg)](https://github.com/yoshinorin/qualtet/actions/workflows/ci.yml)|[![Coverage Status](https://coveralls.io/repos/github/yoshinorin/qualtet/badge.svg?branch=master)](https://coveralls.io/github/yoshinorin/qualtet?branch=master)|
-| `v2.x` |![](https://img.shields.io/badge/Release-v2.21.0-blue.svg?style=flat-square)|[![CI](https://github.com/yoshinorin/qualtet/actions/workflows/ci.yml/badge.svg)](https://github.com/yoshinorin/qualtet/actions/workflows/ci.yml)|[![Coverage Status](https://coveralls.io/repos/github/yoshinorin/qualtet/badge.svg?branch=v2.x)](https://coveralls.io/github/yoshinorin/qualtet?branch=v2.x)|
+| `v2.x` |![](https://img.shields.io/badge/Release-v2.21.0_(stale)-inactive.svg?style=flat-square)|[![CI](https://github.com/yoshinorin/qualtet/actions/workflows/ci.yml/badge.svg?branch=v2.x)](https://github.com/yoshinorin/qualtet/actions/workflows/ci.yml)|[![Coverage Status](https://coveralls.io/repos/github/yoshinorin/qualtet/badge.svg?branch=v2.x)](https://coveralls.io/github/yoshinorin/qualtet?branch=v2.x)|
 | `v1.x` |![](https://img.shields.io/badge/Release-v1.13.0_(stale)-inactive.svg?style=flat-square)|[![CI](https://github.com/yoshinorin/qualtet/actions/workflows/ci.yml/badge.svg?branch=v1.x)](https://github.com/yoshinorin/qualtet/actions/workflows/ci.yml)|[![Coverage Status](https://coveralls.io/repos/github/yoshinorin/qualtet/badge.svg?branch=v1.x)](https://coveralls.io/github/yoshinorin/qualtet?branch=v1.x)|
 
 ## Related Projects
@@ -88,7 +88,7 @@ Please see [releases page](./docs/_docs/releases/index.md).
 
 ## Branches
 
-The current master branch is for `v2.x`. `v1.x` branch is [here](https://github.com/yoshinorin/qualtet/tree/v1.x). But it will be not updated.
+The current master branch is for `v3.x`. The `v2.x` branch is [here](https://github.com/yoshinorin/qualtet/tree/v2.x) and the `v1.x` branch is [here](https://github.com/yoshinorin/qualtet/tree/v1.x). But they will be not updated.
 
 ## License