Allow /56 subnet for ipv6 pool

[olethanh]

Needed for some crns

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 63.34%. Comparing base (6055ad6) to head (a242823).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aleph/vm/network/hostnetwork.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #761   +/-   ##
=======================================
  Coverage   63.34%   63.34%           
=======================================
  Files          77       77           
  Lines        6848     6848           
  Branches      568      568           
=======================================
  Hits         4338     4338           
  Misses       2329     2329           
  Partials      181      181           

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