Moving from macOS 10.13 to a newer version

While trying to upgrade some packages on the mac ci (black tower), I got this message today:

Warning: You are using macOS 10.13.
We (and Apple) do not provide support for this old version.
You will encounter build failures with some formulae.
Please create pull requests instead of asking for help on Homebrew's GitHub,
Twitter or any other official channels. You are responsible for resolving
any issues you experience while you are running this
old version.

We need to consider an OS upgrade for both machines. This should be OK as we will still be able to compile for an older version of macOS as we currently do.

I propose we bump the OS directly to version 10.15 (Catalina).