stack.yaml: use 18.5 LTS until there is a GHC 8.10.6 Docker image

Dockerfile

 # should match the GHC version of the stack.yaml resolver
 # checked in CI
-ARG GHC_VERSION=8.10.6
+ARG GHC_VERSION=8.10.4
 
 FROM haskell:${GHC_VERSION} AS ihaskell_base
 

stack.yaml

 # the GHC version of this resolver needs to match the GHC version in Dockerfile
-resolver: lts-18.7
+resolver: lts-18.5
 allow-newer: true
 
 flags: {}