Move the Ubuntu gcc 4.8 workflow to it's own workflow file

[DavisVaughan]

See #279 (comment)

We try hard to test against gcc 4.8, and one of the reasons is so we ensure that we are compatible with CentOS 7, which uses that compiler version. It would probably be better in the long term to switch to a workflow that uses docker image that uses CentOS 7 to better reproduce the environment we are trying to be compatible with. That would be easiest to do by putting the workflow in its own separate file.

Arrow does this, and they use a rocker image for it, so maybe we could build off that
https://github.com/apache/arrow/blob/f2bac9b63987b63f04a513791517e6c127d13051/.github/workflows/r.yml#L122

[DavisVaughan]

See #359