Fix bad avScan migration leaving legacy object rather than array

[PE39806]

Since migrating File.avScan to its own Scan mongo Document, migration script 009 fails due to using .get on a lean object (POJO i.e. not a hydrated Document) within an async forEach loop which is not awaited, resulting in 009 appearing to pass when it has failed. 012 would then fail as it expected File.avScan to be an array or undefined but was getting objects which it couldn't handle.

This PR fixes these problems by:

• making the find in 009 no longer use lean
• converting the async forEach loop to for ... of in 009
• adding a backpatch for 009 into 012
• making 012 save with a potential Mixed mongoose SchemaType by explicitly marking as modified