Allow backup to complete on old FW versions

[puddly]

Older firmwares do not expose the network key frame counter and thus are unusable. Many people are unfortunately still using old firmwares so instead of breaking their installations, we should allow backup/restore to succeed by picking a "safe" value for the network key frame counter.

I've chosen 1/4 of the maximum value, so a little over 1 billion. It's unlikely many people heavily using Zigbee would approach it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.79%. Comparing base (374c67d) to head (4169b95).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #268   +/-   ##
=======================================
  Coverage   98.79%   98.79%           
=======================================
  Files           7        7           
  Lines         914      916    +2     
=======================================
+ Hits          903      905    +2     
  Misses         11       11           

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