Merge branch '6.0' into 7.0

# Conflicts:
#	ChangeLog/6.0.15_Z_Final.txt
#	Version.props

Author: alex-kulakov

ChangeLog/6.0.15_Z_Final.txt

@@ -0,0 +1,11 @@
+[main] Addressed certain issues with nullable DateTimeOffset values 
+[main] SelectMany with result selector is explicitly not supported when applied after grouping (like .GroupBy(groupingFunc).SelectMany(grouping, selector))
+[main] Addressed issue of inability to delete colums (including such on rename or change type via column recreation) because of undeleted foreign key
+[main] Improved compatibility with non-windows runtimes
+[main] Fixed inability to detect certain parameterizable parts within cached queries (Query.Execute()/.ExecuteDelayed()/ExecuteFuture() groups of methods)
+[mysql] Fixed milliseconds extraction for DateTime expressions in queries
+[mysql] Proper support for milliseconds v5.6+, already created columns will remain the same and new ones will have datetime(6) native type
+[mysql] Improved compatibility with non-windows runtimes
+[postgresql] Improved compatibility with non-windows runtimes
+[bulkoperations] Fixed certain cases of wrong update statements for Firebird provider
+[reprocessing] Improved compatibility with non-windows runtimes