dhall-json bound on aeson can be relaxed

[lucasdicioccio]

Hi there.

While in the process of upgrading my blog engine to 9.8.2, which depends on dhall-json I successfully compiled my engine with an increased bound on aeson for dhall-json (cf. successful cabal.project.local). The build used aeson-2.2.1.0-e989073bc96c18c6c73e8169dee681b2db7807e9177d54bb6d7fb5986cb3e18c on my machine. I have mid-term plans to publish my blog-engine to Hackage so I'm in no rush but I'd like to have done an upgrade/release of a number of my packages by this summer.

I can make the PR if you prefer. Thanks.