revert `diffReport.old|neu` -> `diffReport.snapshot`

[samuelstroschein]

Context

https://github.com/opral/monorepo/pull/3043 was introduced because plugin.applyChanges() did not exist yet. At the time, I thought that lix needs awareness of create, update, delete to apply changes. That turned out to be wrong. A plugin is solely responsible for applying changes, making the PR redundant. Similar insight as LIX-108.

Proposal

Revert https://github.com/opral/monorepo/pull/3043.

• Lix doesn't need to account for deletions. A plugin does with applyChanges().
• Old & neu makes diffing clunky with no added benefit.

[samuelstroschein]

Cancelling because I am confident that we want LIXDK-107