Pull request for Issue #22441 (and also #22690 ) - see those issues for testing instructions.

Summary of changes

We were incorrectly using the lastRowNum property inside of the subformRepeatable object due to
legacy reasons (the logic using a lastRowNum property has been overtaken from history, see commit
f2a9874 ).

What happened is that we always were incrementing the lastRowNum property by 1 inside of the
fixUniqueAttributes method, whilest we call that method multiple times recursively, when we have
nested subforms. So having a 2-level subform meant we would increment the lastRowNum twice for every
added row, effectively meaning we would skip certain indices and hence totally lose the connection
between the actual count of existing rows and their index. Actually we do not need the lastRowNum
property, as the current logic works well without it just by calculating the count of rows directly
in the addRow method. So removing the lastRowNum property is the best solution.

@gaelicwinter and @dawe78 can you please this PR? Thanks!