Support ghc 9.12

[erikd]

Also bump upper bound on hashable dependency.

Currently needs the following in a cabal.project file:

allow-newer:
  , hashable:base
  , splitmix:base
  , tagged:template-haskell
  , vector-stream:ghc-prim

[brandonchinn178]

@treeowl Could we merge this?

I believe this would also close #490

cc @Bodigrim

[Bodigrim]

Given that there were no developments in master since the release of unordered-containers-0.2.20, let me use my Hackage Trustee powers and make a revision, because otherwise it blocks lots of work:

https://hackage.haskell.org/package/unordered-containers-0.2.20/revisions/

@treeowl hopefully it's alright with you. I checked that the test suite compiles and passes. It would be great if you could consider assigning more maintainers to take care of business-as-usual support.

[treeowl]

@Bodigrim, I was taking care of my mother for the last eight months. She died last week, so I should have some more time for software activities. But note that @sjakobi has been more active in this project lately than I have.

[Bodigrim]

My deepest condolences to you, David, and I'm terribly sorry for bad timing on my part. Take care and take your time.

I have not seen @sjakobi around for a while, unfortunately.