Allow DataGraphs v0.5 (backport to release-0.21) (#376)

## Summary

- Widens the `DataGraphs` compat entry to `0.4.1, 0.5` in both the
package and test `Project.toml`, and bumps the version to `0.21.3`. This
backports to the `release-0.21` line the DataGraphs 0.5 support added on
`main` in #374.
- Adds `release-0.21` to the branches that trigger the Registrator
workflow, so this (and future `release-0.21` patch releases) register
automatically on merge, the same way merges to `main` do.
- The full test suite passes locally against DataGraphs 0.5.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mtfishman

.github/workflows/Registrator.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - "master"
       - "main"
+      - "release-0.21"
     paths:
       - "Project.toml"
   issue_comment:

Project.toml

@@ -1,6 +1,6 @@
 name = "ITensorNetworks"
 uuid = "2919e153-833c-4bdc-8836-1ea460a35fc7"
-version = "0.21.2"
+version = "0.21.3"
 authors = ["Matthew Fishman <mfishman@flatironinstitute.org>, Joseph Tindall <jtindall@flatironinstitute.org> and contributors"]
 
 [workspace]
@@ -54,7 +54,7 @@ Adapt = "4"
 Combinatorics = "1"
 Compat = "3, 4"
 ConstructionBase = "1.6"
-DataGraphs = "0.4.1"
+DataGraphs = "0.4.1, 0.5"
 Dictionaries = "0.4"
 Distributions = "0.25.86"
 DocStringExtensions = "0.9"

test/Project.toml

@@ -39,7 +39,7 @@ AbstractTrees = "0.4.5"
 Adapt = "4.3"
 Aqua = "0.8.11"
 Compat = "4.16"
-DataGraphs = "0.4.1"
+DataGraphs = "0.4.1, 0.5"
 Dictionaries = "0.4.4"
 Distributions = "0.25.118"
 Glob = "1.3.1"