Per the [Go Release Policy][1], upstream supports
only the two most recent major releases of Go.
For example, with Go 1.21 available, only 1.21 and 1.20 are supported.
Many projects adopt a similar support policy
to strike a balance between new features and backwards support.
[1]: https://go.dev/doc/devel/release#policy
If the project has the same policy,
it makes sense to test against both versions of Go.
This change adds a test matrix to CI,
testing against both Go 1.20 and 1.21,
on both Windows and Linux.
Abhinav Gupta
de00f3b71c