hls-graph: add lower bound for async (#3021)

`Control.Concurrent.Async` is not available before `async-2.0`

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: Bodigrim

hls-graph/hls-graph.cabal

@@ -61,7 +61,7 @@ library
   hs-source-dirs:     src
   build-depends:
     , aeson
-    , async
+    , async >= 2.0
     , base >=4.12 && <5
     , bytestring
     , containers