fix #1962, drop dfg19 compat

Author: dehann

Project.toml

@@ -50,7 +50,7 @@ ApproxManifoldProducts = "0.6.3"
 BSON = "0.2, 0.3"
 Combinatorics = "1.0"
 DataStructures = "0.16, 0.17, 0.18"
-DistributedFactorGraphs = "0.19.3, 0.20"
+DistributedFactorGraphs = "0.20"
 Distributions = "0.24, 0.25"
 DocStringExtensions = "0.8, 0.9"
 FileIO = "1"