logging: depend on LOG_MODE_IMMEDIATE

[mniestroj]

With Zephyr logging v2 there was an overhaul of Kconfig options. At first old options were supported to some point. Right now there is no LOG_IMMEDIATE, which LOG_BACKEND_GOLIOTH depended on.

Change LOG_IMMEDIATE to LOG_MODE_IMMEDIATE. This makes sure that for platforms with synchronous logging by default (like recently introduced native_sim in upstream Zephyr) Golioth logging backend doesn't get enabled.

[github-actions[bot]]

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