Please be aware that GCC-7 is no longer supported on Ubuntu 20.04 w.r.t. build compatibility

[tangxifan]

Due to the upgrades in VTR (see details in #1439), we have to drop the build compatibility support on GCC-7 for Ubuntu 20.04.

Even though we are trying our best to maintain build compatibility, it is very challenging as OS version is advancing and Github will drop out-of-date LTS version of OS.

We will consider to migrate our CI tests from Ubuntu 20.04 to Ubuntu 22.04 in 2024.

If there is any conflicts, feel free to share your opinions.