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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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