SQL Server AlwaysOn database stuck in Not Synchronizing / In Recovery mode after upgrading. Error: Cannot open database '...' version 782
After poking around in SSMS for a while I noticed that on the secondary replica there was a pause icon next to the Availability Databases. The primary had shown both were "green", but there was an option on the secondary to Resume Data Movement. I resumed the first database, and immediately the In Recovery status message was removed. A minute later it changed from Not Synchronizing to Synchronized, and everything worked as expected.

Here is a screenshot of the AG Databases after I fixed "Patch", but before fixing the test database:

 

Note you can also use TSQL on the secondary to resume replication on multiple database at the same time:

ALTER DATABASE [Patch] SET HADR RESUME;
ALTER DATABASE [test] SET HADR RESUME;
GO