Remove ubuntu-18.04 runner image dependency from CI pipeline.

[lalitb]

ubunu-18.04 runner images won't be supported from April 1st 2023 as mentioned here - actions/runner-images#6002.

ubuntu-18.04 is used in CI for testing the otel-cpp builds with GCC 4.8 compiler (the initial GCC compiler with C++11 support). As we won't have C++11 support dropped before April, it would be better to upgrade the CI pipeline. The quick fix would be to upgrade to Ubuntu 20.04, and work around the lack of GCC 4.8 by manually adding the package repositories from Xenial.

[marcalff]

This explains the failure in CI with GCC 4.8:

To raise awareness of the upcoming removal, we will temporarily fail jobs using Ubuntu 18.04. Builds that are scheduled to run during the brownout periods will fail.

[marcalff]

The quick fix would be to upgrade to Ubuntu 20.04, and work around the lack of GCC 4.8 by manually adding the package repositories from Xenial.

Do we really need GCC 4.8, or just to compile with C++11 ?

How about compiling with a more recent GCC (available on Ubuntu 20.04), with -std=c++11 instead of the default ?

[ThomsonTan]

The quick fix would be to upgrade to Ubuntu 20.04, and work around the lack of GCC 4.8 by manually adding the package repositories from Xenial.

Do we really need GCC 4.8, or just to compile with C++11 ?

How about compiling with a more recent GCC (available on Ubuntu 20.04), with -std=c++11 instead of the default ?

I remember GCC 4.8 is still used by @owent .
#1830 (comment)

[lalitb]

Do we really need GCC 4.8, or just to compile with C++11 ?

There is special handling in the code for some of the scenarios where GCC 4.8 fails with it's C++11 support (regex being one of them). But yeah, if it's not easy to use GCC 4.8 over ubuntu 20.04, we can switch to -std=c++11. Let me give it a try :)