go/oasis-node: Bump default pruning config to 250_000 versions

[martintomazic]

Let's use a sane defaults for the pruning config (context).

Update defaults to 250_000 versions, so that node has enough consensus and runtime state to create at least two chekpoints if the checkpoint interval is 100_000. At the same time this is still far from the number of versions, that makes Sapphire pruning prohibitively slow.

Follow-ups

• In the future to make code more robust we might consider prune handlers that prevent pruning past pending checkpoint creation. (out of scope).
• Enable checkpoint creation by default to ensure long term data availability of the checkpoints.
• Probably we should enable pruning by default.

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[martintomazic]

Bumping default configs might break configs that rely on the default values, so should the changelog be breaking? In practice the default value of 600 was already not practical.

[martintomazic]

CI was green meaning no e2e test relied on this defaults. :)