centos7等老版本的国内源配置

centos7的老版本，在 yum update的时候，无法执行。可以用如下的命令解决：

# 1. 清理并备份旧配置
mkdir -p /etc/yum.repos.d/bak
mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/bak/

# 2. 写入清华大学 TUNA 归档源配置
cat << 'EOF' > /etc/yum.repos.d/CentOS-Base.repo
[base]
name=CentOS-7.9.2009 - Base - mirrors.tuna.tsinghua.edu.cn
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/7.9.2009/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[updates]
name=CentOS-7.9.2009 - Updates - mirrors.tuna.tsinghua.edu.cn
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/7.9.2009/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[extras]
name=CentOS-7.9.2009 - Extras - mirrors.tuna.tsinghua.edu.cn
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/7.9.2009/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[centosplus]
name=CentOS-7.9.2009 - Plus - mirrors.tuna.tsinghua.edu.cn
baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos-vault/7.9.2009/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
EOF

# 3. 刷新缓存
yum clean all
yum makecache