Document/Update s2n to build process for Ubuntu 20.04

[alexw91]

Problem:

s2n currently builds on older versions of Ubuntu. There are several modifications to the existing s2n build that are needed to get s2n to build on Ubuntu 20.04.

Issue 1: Ubuntu 20.04's openssl.cnf file disables TLS 1.0, which breaks s2n's Python integration tests.

Potential Fixes:

1. Manually edit the installed openssl.cnf file to allow TLS 1.0 system wide.
   - Link: https://stackoverflow.com/a/61568390
2. Create and Maintain our own openssl.cnffile to be used during Integration Tests

Issue 2: Older Compiler versions are not available on Ubuntu 20.04

Potential Fix:

1. Allow Ubuntu to install Apt packages from older Ubuntu versions in order to install older compiler versions:
   - Link: https://gist.github.com/zuyu/7d5682a5c75282c596449758d21db5ed#gistcomment-3807040

Issue 3: GnuTLS build scripts do not complete on Ubuntu 20.04

Potential Fix: N/A, needs investigation

Solution:

Make updates to the s2n build scripts to work on Ubuntu 20.04

[dougch]

Gnutls was fixed as part of the move to integrationv2. We have a pre-built docker container for Ubuntu22 running the fuzz tests, but as mentioned above, there are still missing tools that are not available. Our approach now will be to get the integration tests running with nix.