Deprecate download utilities

[wpbonelli]

Can we substitute the github CLI (or maybe one of the Python clients, though there is some risk adopting a 3rd party project) for the utilities in the download.py module? These are used

• extensively in the MF6 release automation
• for the flopy MF6 code generation utility

Pymake also has a similar module, which the devtools module grew out of and which we discussed replacing with devtools eventually. It would be nice not to have to maintain API client code.

[wpbonelli]

Recently discussed permanently rehoming the pymake download stuff here, along with some other things (e.g. #243). Still need to consider whether a 3rd party library might be able to help, or if we can just use the github CLI for everything