3.6.0: JSON templating change release notes & test cases

# Summary

#12909 made some fairly fundamental changes to JSON/struct usage in templates, so this needs to be documented in the release notes, per https://github.com/argoproj/argo-workflows/pull/12909#issuecomment-2285300935

There were also a few cases in particular that need testing for breaking changes.

## Use Cases

- [x] Release Notes
- [ ] Test cases
- [x] #8930 -- `toJSON` + `jsonpath` on struct
- [x] #12739 -- `toJSON` + `fromJSON` on struct
- [ ] Other test cases on expressions on structs?
- [ ] Other test cases for this change in general?

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