Mark v0.7.1 as unmaintained and v0.7.3 as maintained (#139)

### What this PR does / why we need it?
Mark v0.7.1 as unmaintained and v0.7.3 as maintained:
vLLM released the v0.7.3 version:
https://github.com/vllm-project/vllm/releases/tag/v0.7.3 which include
serval commits:
- https://github.com/vllm-project/vllm/pull/12874
- https://github.com/vllm-project/vllm/pull/12432
- https://github.com/vllm-project/vllm/pull/13208

We'd better to bump the versions to v0.7.3.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Preview

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>

README.zh.md

@@ -78,6 +78,13 @@ vllm-ascend有主干分支和开发分支。
 - **main**: 主干分支，与vLLM的主干分支对应，并通过昇腾CI持续进行质量看护。
 - **vX.Y.Z-dev**: 开发分支，随vLLM部分新版本发布而创建，比如`v0.7.1-dev`是vllm-asend针对vLLM `v0.7.1`版本的开发分支。
 
+下面是维护中的分支：
+
+| 分支         | 状态         | 备注                  |
+|------------|------------|---------------------|
+| main       | Maintained | 基于vLLM main分支CI看护   |
+| v0.7.3-dev | Maintained | 基于vLLM v0.7.3版本CI看护 |
+
 请参阅[版本策略](docs/source/developer_guide/versioning_policy.zh.md)了解更多详细信息。
 
 ## 贡献