Update for Stack LTS 21.9 and Aeson 2.x

[ericpashman]

This pull request updates the package to work with the latest Stack LTS resolver.

This change has three main components:

• update stack.yaml to specify the LTS 21.9 resolver and eliminate explicit version bounds on most dependencies;
• delete several orphan instances that are redundant of instances exported by newer dependencies; and
• update the codebase for compatibility with Aeson's KeyMap type (introduced in v2.0.0.0).

The package builds successfully with stack build.

There are a few outstanding issues:

• The json-autotype-gen-test test suite fails. I suspect this was already the case, but I don't know because I can't get the package to build without my updates. The test suit is opaque, so I can't see why it's failing.
• The package does not build with cabal build, failing immediately with a parse error of the cabal.project file. I didn't look into this, but I suggest deleting all of the Cabal-specific files and (if desired) re-generating them from the working Stack config with a tool like stack2cabal.
• Hpack decremented the version number of the json-alt package in its json-alt.cabal file from 1.0.1 to 1.0.0., presumably because someone in the past edited the Cabal file directly rather than the Hpack package.yaml file to bump the version number. Hpack doesn't offer much at this point aside from incidental complexity, so I would suggest again generating Cabal files from the working Stack config, and getting rid of the Hpack config files altogether.
• I didn't bump the version numbers on any of the packages.

If you interested in merging this, let me know what you'd like done to get it in a state you'd be happy with.

[FranklinChen]

Since this package seems to no longer be maintained by the original author, what is the protocol to keep it going?