dev process: add label for NS7 issues (#6746)

Co-authored-by: Davide Principi <[email protected]>

Author: gsanchietti

developer-manual/development_process.rst

@@ -124,6 +124,9 @@ Issues can be tagged using a set of well-known labels:
 
 An issue without a label is considered a new feature or an enhancement.
 
+Until June 2024, both NethServer 7 (NS7) and NethServer 8 (NS8) are actively supported.
+You can use the `ns7` label label to mark the issues for NethServer 7.
+
 Before introducing new labels, please discuss them with the development team
 and make sure to describe carefully the new label inside the `label page <https://github.com/NethServer/dev/labels>`_.