build(fpm): update dag dependency

This commit updates to version 2.1.0 of dag, which
exploits doubly nested concurrency (do concurrent)
in the topological_sort function.

Author: Damian Rouson

fpm.toml

@@ -7,7 +7,7 @@ copyright = "2021 Sourcery Institute"
 
 [dependencies]
 assert = {git = "https://github.com/sourceryinstitute/assert", tag = "1.3.0"}
-dag = {git = "https://github.com/sourceryinstitute/dag", tag = "2.0.1"}
+dag = {git = "https://github.com/sourceryinstitute/dag", tag = "2.1.0"}
 
 [dev-dependencies]
 vegetables = {git = "https://gitlab.com/everythingfunctional/vegetables", tag = "v7.0.2"}