Compatibility with Stackage Nightly

[ysangkok]

This makes the Aeson 1.5 series compile with the latest Nightly, which uses GHC 9.2.

One test is failing, as can be seen with

stack test --stack-yaml stack-nightly.yaml --test-arguments '-p tests.unit.FromJSONKey'

But this test is failing even if base branch is tested on the lts-19 snapshot
(which uses GHC 9.0 and doesn't need these patches). So I think this is a
separate concern. That issue can be seen with just this stack-nightly.yaml change applied to the aeson-1.5 branch:

diff --git a/stack-nightly.yaml b/stack-nightly.yaml
index fc76b6b..2bc276b 100644
--- a/stack-nightly.yaml
+++ b/stack-nightly.yaml
@@ -1,4 +1,4 @@
-resolver: nightly-2019-10-14
+resolver: lts-19.0
 packages:
 - '.'
 - attoparsec-iso8601
@@ -7,5 +7,3 @@ flags:
     fast: true
   attoparsec-iso8601:
     fast: true
-extra-deps:
-- generic-deriving-1.12.4

[ysangkok]

This continues in the spirit of ae04b00 by @phadej. I am not sure why that commit didn't also add the #ifs seen in this PR.

[ysangkok]

@bergmark Would it be possible to release this somehow as you alluded to in #932 (comment) ? I could reopen this if you'd be open for that.