Reliable data file upgrades

[AArnott]

Add tests for data file upgrades and fix bugs found thereby.

[codecov-commenter]

Codecov Report

Merging #93 (f117ec9) into main (02f2685) will decrease coverage by 1.20%.
The diff coverage is 12.44%.

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
- Coverage   41.96%   40.75%   -1.21%     
==========================================
  Files          91       92       +1     
  Lines        6451     6657     +206     
==========================================
+ Hits         2707     2713       +6     
- Misses       3744     3944     +200     

Impacted Files	Coverage Δ
...oneyManagement.Tests/DatabaseSchemaManagerTests.cs	0.00% <0.00%> (ø)
...t/Nerdbank.MoneyManagement.Tests/MoneyFileTests.cs	0.00% <0.00%> (ø)
.../Nerdbank.MoneyManagement/DatabaseSchemaManager.cs	85.45% <90.00%> (ø)
src/Nerdbank.MoneyManagement/MoneyFile.cs	94.49% <100.00%> (+0.04%)	⬆️
...Management/ViewModels/InvestingAccountViewModel.cs	87.50% <0.00%> (+1.13%)	⬆️

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