feat: refactor: remove NewestNetworkVersion

[arajasek]

Related Issues

This field has always been terrible (see TODO above it), and is potentially confusing to users (see #9319 (comment)).

Proposed Changes

Remove its only usage in source code, which is in the messagepool -- just query for the NetworkVersion when needed. If we have concerns that this is slow, we could just set it once at startup, and use that -- it is very rare for this to matter (hasn't mattered since nv7).

Rename remaining usages to TestNetworkVersion.

Additional Info

Checklist

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  • example: fix: mempool: Introduce a cache for valid signatures
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• This PR has tests for new functionality or change in behaviour
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