2 spaces seems to be the more common indentation level so that's what
I unified around.

We still want to build VS 2017 through AppVeyor, and those images
have CMake 3.16.2 installed. We could install newer CMake as part
of the build, but since we don't use newer CMake features yet, this
is simpler.

We also bump the minimum CMake version to 3.20 as per #2943

The main reason for this is to be able to distinguish between
different errors (or "errors") based on the return code. Before
this change, it was impossible to use the exit code to figure out
whether a test binary failed because all tests were skipped or
because exactly 4 assertions have failed.

This meant that using `catch_discover_tests` and telling it to
check for exit code == 4 to determine skipped tests could lead to
false negatives.

Previously it could be just plain reporter name, e.g. `xml`, but
it could not specify other reporter options. This change is not
particularly useful for the built-in reporters, as it mostly comes
in handy for combining specific custom reporter with custom arguments,
and the built-in reporters do not have those.

Signed-off-by: Cristian Le <cristian.le@mpsd.mpg.de>