SI-6963 Deprecates -Xmigration switch

-Xmigration is specific to the 2.7 to 2.8 upgrade and is no longer
relevant. There is no plan to maintain it so it will be removed. This
commit deprecates it in anticipation.
Merge pull request #1895 from JamesIry/SI_6963_2.10.x

SI-6963 Deprecates -Xmigration switch