Fix (nearly) endless database upgrade dialogs

[soininen]

This PR fixes a bug where multiple database upgrade dialogs would pop up for a single Data store if user kept cancelling the dialogs. We now ask just once if an upgrade is desirable, though users are given a second chance if they try to open the DB in the editor.

Fixes #2915

Checklist before merging

• [x] Documentation is up-to-date
• [x] Release notes have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black & isort
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 66.94%. Comparing base (deb9cbe) to head (57fa23a).

Files	Patch %	Lines
spinetoolbox/spine_db_manager.py	60.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2916 +/- ## ======================================= Coverage 66.93% 66.94% ======================================= Files 189 189 Lines 36887 36890 +3 Branches 6314 6314 ======================================= + Hits 24690 24695 +5 + Misses 10884 10883 -1 + Partials 1313 1312 -1 ```

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