Fix for default preset EU/UK (Narrow)

[formtapez]

In #1820 the default preset was changed to EU/UK (Narrow), but without setting LORA_CR to 8.
The coding rate actually defaults to 5, so that it doesn't match the preset.
These settings are compatible to each other, but the lower coding rate is not very reliable.

[Confituurke]

+1 for this

Right now, a lot of tooling classifies nodes on stock defaults as a custom preset (869.618 / 62.5 / SF8 / CR5) instead of matching EU/UK (Narrow) (869.618 / 62.5 / SF8 / CR8), which makes maps and presets misleading.

For a mesh we can rely on, every node on that channel should use the same LoRa modem settings (frequency, bandwidth, spreading factor, and coding rate - not only the first three).

We are already seeing fragmented/degraded meshes in Western Europe because of this split, and it was hard to debug because a CR mismatch is easy to miss next to the correct frequency, BW, and SF.

I am surprised this was not raised sooner.

Unless there is evidence that CR5 is deliberately better for that EU narrow air setting, we should ship this fix in the next release, otherwise the honest alternative is to change the official EU/UK (Narrow) preset (and API copy) to CR5 so documentation, apps, and firmware all match + sending out a message to the whole MeshCore community.