Ubuntu执行sudo apt-get update报错及软件更新器无法检测网络的问题求助

大家好，我最近在Ubuntu系统上碰到了两个头疼的问题，想请各位帮忙看看怎么解决：

1. sudo apt-get update 执行报错 #

我运行sudo apt-get update时，出现了域名解析失败的错误，没法访问ro.old-releases.ubuntu.com这个源地址，而且我现在不知道该怎么恢复系统默认的软件源。以下是终端的完整输出：

Err:5 http://ro.old-releases.ubuntu.com/ubuntu jammy-updates InRelease
Could not resolve 'ro.old-releases.ubuntu.com'

Err:6 http://ro.old-releases.ubuntu.com/ubuntu jammy InRelease
Could not resolve 'ro.old-releases.ubuntu.com'

Err:7 http://ro.old-releases.ubuntu.com/ubuntu jammy-backports InRelease
Could not resolve 'ro.old-releases.ubuntu.com'

Reading package lists... Done

N: Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'https://brave-browser-apt-release.s3.brave.com stable InRelease' doesn't support architecture 'i386'

W: Failed to fetch http://ro.old-releases.ubuntu.com/ubuntu/dists/jammy-updates/InRelease  Could not resolve 'ro.old-releases.ubuntu.com'

W: Failed to fetch http://ro.old-releases.ubuntu.com/ubuntu/dists/jammy/InRelease  Could not resolve 'ro.old-releases.ubuntu.com'

W: Failed to fetch http://ro.old-releases.ubuntu.com/ubuntu/dists/jammy-backports/InRelease  Could not resolve 'ro.old-releases.ubuntu.com'

W: Some index files failed to download. They have been ignored, or old ones used instead.

2. 软件更新器无法检测到网络连接 #

除此之外，系统自带的软件更新器也出了问题——我明明已经连接上网络了，而且系统用的是默认设置，没开启任何VPN或其他可能影响网络的软件，但它就是检测不到网络连接，弹出了报错提示。报错截图如下：

这些问题都是在我给系统更新了最新的系统信息之后才出现的，麻烦各位大佬帮忙看看怎么解决，非常感谢！

备注：内容来源于stack exchange，提问作者user3277459