stack: Switch to lts-13.10 resolver

According to stackage.org, 13.11 should be the last LTS release with GHC 8.6.3,
but strangely that also uses 8.4.4. The now selected 13.10 should really depend
the older GHC.

stack.yaml

-resolver: lts-13.11
+resolver: lts-13.10
 
 packages:
 - .