Maintenance release GAO 1.6 for changes from ph-scale 1.6 RC testing

[zepumph]

We want to a maintenance release for GAO 1.6 to make sure we have a consistent PhET-iO product. Here is what @samreid and I recommend:

• In https://github.com/phetsims/phet-io-wrappers/issues/483 we will make sure that it works as an "old version" of the version migration wrapper.
• ~For a more full migration support, we need the "ConvertToStringsStateForLocale" rule in https://github.com/phetsims/joist/issues/899.~ This would be too hard, and not enough gain.
• We want LinkedElements to be stateful, from https://github.com/phetsims/phet-io/issues/1903.

[samreid]

It doesn't feel important to do this eagerly (wouldn't save much time or increase product value in the long run). I'll demote priority for now, but others feel free to disagree.