Support for `IF EXISTS` in SQL Server 2016 (13.x) and later versions

Support for `IF EXISTS` in SQL Server 2016 (13.x) and later versions

Added support for the ``IF EXISTS`` clause when dropping indexes on SQL
Server 2016 (13.x) and later versions. The :paramref:`.DropIndex.if_exists`
parameter is now honored by the SQL Server dialect, allowing conditional
index drops that will not raise an error if the index does not exist.
Pull request courtesy Edgar Ramírez Mondragón.

Fixes: #13045
Closes: #13046
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/13046
Pull-request-sha: 65dca2055cb403430730b5cf42f0c5f55b23bfb1

Change-Id: Iab95b1a46003b38709a791b8a7c4233dfda5e830