解决Ubuntu18.10Cosmic更新源问题

### 背景
最近发现 Ubuntu 18.10 Cosmic 的 `apt update` 命令无法正常使用。经过调查，了解到 Ubuntu 的各个版本都有一定的支持期限，一旦超过这个期限，系统会自动切换到旧版本库（old releases）。

### 尝试更换中科大源
首先，我尝试将软件源更换为中科大的镜像源，以提高更新速度和稳定性。以下是更换后的源配置：

```bash
# 中科大源
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-proposed main restricted universe multiverse
```

然而，即使更换了中科大的源，依然出现了 `404 Not Found` 错误。这说明中科大的镜像源也已经停止对 Ubuntu 18.10 Cosmic 的支持。

### 切换到旧版本库
为了继续使用 `apt update`，需要将源切换到旧版本库（old releases）。以下是新的源配置：

```bash
# 中科大旧版本库
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu-old-releases/ubuntu/ cosmic-proposed main restricted universe multiverse
```

保存上述配置后，运行 `sudo apt update`，成功解决了更新问题。

### 参考资料
- Ubuntu 当前支持的版本列表：[Ubuntu Releases](https://wiki.ubuntu.com/Releases)
- 中科大 Ubuntu 镜像帮助文档：[USTC Ubuntu Help](https://mirrors.ustc.edu.cn/help/ubuntu.html)
- 清华大学 Ubuntu 镜像帮助文档：[TUNA Ubuntu Help](https://mirror.tuna.tsinghua.edu.cn/help/ubuntu/)

### 附录：Ubuntu 版本对照表
| 版本 | 开发代号 | 发布日期 | 支持结束时间 |
| --- | --- | --- | --- |
| 4.10 | Warty Warthog | 2004-10-20 | 2006-04-30 |
| 5.04 | Hoary Hedgehog | 2005-04-08 | 2006-10-31 |
| 5.10 | Breezy Badger | 2005-10-13 | 2007-04-13 |
| 6.06 LTS | Dapper Drake | 2006-06-01 | 2009-07-14 (桌面版) / 2011-06-01 (服务器版) |
| 6.10 | Edgy Eft | 2006-10-26 | 2008-04-25 |
| 7.04 | Feisty Fawn | 2007-04-19 | 2008-10-19 |
| 7.10 | Gutsy Gibbon | 2007-10-18 | 2009-04-18 |
| 8.04 LTS | Hardy Heron | 2008-04-24 | 2011-05-12 (桌面版) / 2013-05-09 (服务器版) |
| 8.10 | Intrepid Ibex | 2008-10-30 | 2010-04-30 |
| 9.04 | Jaunty Jackalope | 2009-04-23 | 2010-10-23 |
| 9.10 | Karmic Koala | 2009-10-29 | 2011-04-30 |
| 10.04 LTS | Lucid Lynx | 2010-04-29 | 2013-05-09 (桌面版) / 2015-04-30 (服务器版) |
| 10.10 | Maverick Meerkat | 2010-10-10 | 2012-04-10 |
| 11.04 | Natty Narwhal | 2011-04-28 | 2012-10-28 |
| 11.10 | Oneiric Ocelot | 2011-10-13 | 2013-05-09 |
| 12.04 LTS | Precise Pangolin | 2012-04-26 | 2017-04-28 (桌面版) / 2019-04 (服务器版) |
| 12.10 | Quantal Quetzal | 2012-10-18 | 2014-05-16 |
| 13.04 | Raring Ringtail | 2013-04-25 | 2014-01-27 |
| 13.10 | Saucy Salamander | 2013-10-17 | 2014-07-17 |
| 14.04 LTS | Trusty Tahr | 2014-04-17 | 2019-04 (桌面版) / 2024-04 (服务器版) |
| 14.10 | Utopic Unicorn | 2014-10-23 | 2015-07-23 |
| 15.04 | Vivid Vervet | 2015-04-23 | 2016-02-04 |
| 15.10 | Wily Werewolf | 2015-10-22 | 2016-07-28 |
| 16.04 LTS | Xenial Xerus | 2016-04-21 | 2021-04 (桌面版) / 2024-04 (服务器版) |
| 16.10 | Yakkety Yak | 2016-10-13 | 2017-07-20 |
| 17.04 | Zesty Zapus | 2017-04-13 | 2018-01-13 |
| 17.10 | Artful Aardvark | 2017-10-19 | 2018-07-19 |
| 18.04 LTS | Bionic Beaver | 2018-04-26 | 2023-04 (桌面版) / 2028-04 (服务器版) |
| 18.10 | Cosmic Cuttlefish | 2018-10-18 | 2019-07 |
| 19.04 | Disco Dingo | 2019-04-18 | 2020-01 |