build: target Windows 7 when building libevent and fix ipv6 usage

Summary:
Fix for ipv6 on Windows.

Backport of core PR19375.

Depends on D7742.

Test Plan:

make build-win64

cmake -GNinja .. \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake \
  -DBUILD_BITCOIN_SEEDER=OFF
ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7743