chore: release 0.21.1 (update CITATION.cff)

Author: github-actions[bot]

CITATION.cff

@@ -3,7 +3,7 @@ message: "If you use this software, please cite it as below."
 
 title: "SysML2.NET: A .NET implementation of the OMG SysML v2 specification"
 abstract: "SysML2.NET is a .NET implementation of the OMG SysML2 specification. SysML2.NET provides a number of libraries in the form of a C# SDK."
-version: "0.21.0"
+version: "0.21.1"
 date-released: "2026-06-01"
 url: "https://github.com/STARIONGROUP/SysML2.NET"
 
@@ -31,6 +31,6 @@ preferred-citation:
       family-names: Théate
       affiliation: "Starion Group S.A."
     - name: "Starion Group S.A."
-  version: "0.21.0"
+  version: "0.21.1"
   url: "https://github.com/STARIONGROUP/SysML2.NET"
   date-released: "2026-06-01"