Remove support for deserialization of old `pipeline_outputs` from pipeline snapshot

[Amnah199]

Summary and motivation

When using pipeline breakpoints, Haystack creates pipeline snapshots that include pipeline_outputs. In this PR, we fixed the (de)serialization logic for pipeline_outputs so that snapshots now use the correct serialization schema.

As a result, older snapshots still contain pipeline_outputs in the legacy format, and we currently maintain backward-compatibility logic to deserialize them. This compatibility layer is only temporary and must be removed before the Haystack 2.23.0 release.

Required Action

Remove the backward-compatibility code paths that handle legacy pipeline_outputs serialization.
(Update the code sections here once the PR is merged)

Checklist

Tasks

• The changes are merged in the main branch (Code + Docstrings)
• Release notes have documented the breaking change
• A new version of haystack-ai has been released on PyPI
• Docs at https://docs.haystack.deepset.ai/ were updated
• Integrations on haystack-core-integrations were updated (if needed) - This step might require a Breaking change proposal on the repo
• Notebooks on https://github.com/deepset-ai/haystack-cookbook were updated (if needed)
• Tutorials on https://github.com/deepset-ai/haystack-tutorials were updated (if needed)
• Articles on https://github.com/deepset-ai/haystack-home/tree/main/content were updated (if needed)
• Integration tile on https://github.com/deepset-ai/haystack-integrations was updated (if needed)

[anakin87]

#10249 might implement this. Review the PR after 2.22.0 and before 2.23.0 release.