multicast: use a single map for both ipv4 and ipv6.

[Dirbaio]

Advantages:

• Deduplicates logic a bit
• The "max multicast group count" compile-time config now actually makes sense. Before the actual max was 2n, but only reachable with N ipv4 and N ipv6.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.27%. Comparing base (dc08e0b) to head (4990fb9).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/iface/interface/igmp.rs	78.94%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #976   +/-   ##
=======================================
  Coverage   79.27%   79.27%           
=======================================
  Files          81       81           
  Lines       26918    26926    +8     
=======================================
+ Hits        21339    21346    +7     
- Misses       5579     5580    +1     

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