[Dev Core Manual] - Migration - Explain how obsolete work in the dev manual and update our logic

[Migration] - Explain how obsolete work in the dev manual

The attribute must gain the value OBSOLETE and the attribute must be set as "may" within its objectClass.

This is a key factor for migration tool to work properly.

~~https://fusiondirectory-developer-documentation.readthedocs.io/en/1.4/fusiondirectory/release/releasepolicy.html

We also have to update the documentation for release policy, explaining the removal of obsolete tags between major release.~~

Note : Decision has been made to create an internal documentation about CORE development and guidelines. This issue is to be integrated into that logic and not within public dev-manual