cmake-gui: avoid undefined font in recent macOS

Since 8fb74f97 (ENH: Use fixed pitch font in output window., 2008-03-07) cmake-gui has been using a "Courier" font to print mono spaced text in the output window, but at least in macOS 12, Courier is no longer available and therefore will print the following warning in the invoking terminal:

qt.qpa.fonts: Populating font family aliases took 175 ms. Replace uses of missing font family "Courier" with one that exists to avoid this cost.

Change it to use "Courier New" instead, which has been provided as an automatically selected alternative in Windows and comes preinstalled in both systems, and add a hint to pull another monospaced font if not.

Topic-rename: cmake-gui-font