Ubuntu 18.04 will reach its End of Standard Support in April 2023:
https://wiki.ubuntu.com/Releases
By updating Ubuntu from 18.04 to 20.04, the dynamically-linked glibc
version is bumped up from 2.27 to 2.31.
The dynamically linked containerd binary still seems to be compatible with
CentOS 7 (glibc 2.17).
The runc binary in the `cri-containerd(-cni)-<VERSION>-linux-<ARCH>.tar.gz`
bundle no longer works on CentOS 7, though, but this is acceptable, as the
`cri-containerd(-cni)` bundle has been deprecated since containerd 1.6.
```
$ ldd /usr/local/sbin/runc
linux-vdso.so.1 => (0x00007fffee9c4000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00007eff48721000)
libseccomp.so.2 => /lib64/libseccomp.so.2 (0x00007eff484e0000)
libc.so.6 => /lib64/libc.so.6 (0x00007eff48112000)
/lib64/ld-linux-x86-64.so.2 (0x00007eff492cb000)
$ /usr/local/sbin/runc
/usr/local/sbin/runc: symbol lookup error: /usr/local/sbin/runc: undefined symbol: seccomp_notify_respond
```
Fix issue 7961
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
