docs: Add discontinue statement for this project

[Xuanwo]

Fix https://github.com/alibaba/AliSQL/issues/112

Cc @xpchild

This PR will also:

• Fix https://github.com/alibaba/AliSQL/issues/111
• Fix https://github.com/alibaba/AliSQL/issues/107
• Fix https://github.com/alibaba/AliSQL/issues/102
• Fix https://github.com/alibaba/AliSQL/issues/100
• Fix https://github.com/alibaba/AliSQL/issues/98
• Fix https://github.com/alibaba/AliSQL/issues/96
• Fix https://github.com/alibaba/AliSQL/issues/92
• Fix https://github.com/alibaba/AliSQL/issues/90

---

Reasons for discontinuing:

https://github.com/alibaba/AliSQL/issues/112#issuecomment-1239034070

对于 5.6 的版本：1. 官方已经停止维护 2. 大量客户还在 5.6 版本上使用 3. 历史原因导致 8.0 没有在同一个repo上。 导致我们没有在 5.6上做更新，但又不能关闭。

[CLAassistant]

All committers have signed the CLA.

[BohuTANG]

Former contributor of AliSQL 6 years ago, I suggest setting this repo to archive, cc @xpchild

[Xuanwo]

An empty response is the best discontinue statement, I got it.