`whenExpression`: `when` using `expr` instead of `govaluate`

# Summary

We don't like `govaluate`, we want to get off it, it is confusing to users:

- #7576
- #7509
- #7089
- #6823
- #7382

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**Message from the maintainers**:

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Start by reviewing the related issues #7576, #7509, #7089, #6823, and #7382 to understand the reported problems with govaluate and the desired behavior. Then locate the implementation of whenExpression and its tests. Done means when expressions use expr instead of govaluate without losing the expected behavior described in those issues.

Written by the indexing model from the issue text.