Add Downgrade workflow

[fhagemann]

This addresses #221: the Downgrade workflow builds the package and runs the tests with the lowest compatible version allowed by the Project.toml.

I'm getting errors when running this with StaticArrays versions 0.9.0, 0.12.0 and 1.0.0.
Lowest version that works seems to be 1.4.6, see #221.

[fhagemann]

As pointed out in #221, this might need to go hand in hand with a Compat.toml change in the General registry, e.g.

["0 - 0.4.3"]
Distances = "0.6 - 0.8"
StaticArrays = "0.7 - 0.12"
julia = ["0.7", "1"]

["0.4.14 - 0"]
julia = "1.6.0 - 1"
StaticArrays = "1.4.6"

["0.4.21 - 0"]
Distances = "0.10.12 - 0.10"

["0.4.4"]
Distances = "0.8.1 - 0.8"

["0.4.4 - 0.4.13"]
julia = "1"

["0.4.4 - 0.4.7"]
StaticArrays = "0.9 - 0.12"

["0.4.5 - 0.4.6"]
Distances = "0.9"

["0.4.7 - 0.4.20"]
Distances = "0.9 - 0.10"

["0.4.8 - 0.4.13"]
StaticArrays = ["0.9 - 0.12", "1"]