Project recent_files not respected in Spyder 5

[mrclary]

What steps will reproduce the problem?

One issue I found is that projects opened in Spyder 5 do not respect the recent_files list in .spyproject/workspace.ini, if the project was created in Spyder 4.

It appears the reason is that Spyder 4 puts it under [main] while Spyder 5 looks at [workspace]. The listing under [main] is not clobbered, just ignored.

This means that recent files in projects will need to be reopened. It also means possible pollution or ambiguities of recent_files. Perhaps having Spyder look for it in both places, moving it to [workspace] if found in [main]?

Working with artifact from #15054.