doc: Bump to Ubuntu Bionic 18.04 in build-windows.md
doc: Bump to Ubuntu Bionic 18.04 in build-windows.md

Windows starts to provide Ubuntu 18.04 WSL, it can be downloaded from Microsoft Store.

GitHub-Pull: #13246
Rebased-From: 9d4f942

GroestlCoin/GroestlCoin,1185/starwels,1185/starwels,DigitalPandacoin/pandacoin,GroestlCoin/bitcoin,myriadteam/myriadcoin,DigitalPandacoin/pandacoin,GroestlCoin/bitcoin,viacoin/viacoin,myriadcoin/myriadcoin,myriadteam/myriadcoin,GroestlCoin/GroestlCoin,DigitalPandacoin/pandacoin,ppcoin/ppcoin,gjhiggins/vcoincore,myriadcoin/myriadcoin,GroestlCoin/bitcoin,viacoin/viacoin,peercoin/peercoin,1185/starwels,gjhiggins/vcoincore,myriadteam/myriadcoin,GroestlCoin/bitcoin,viacoin/viacoin,myriadcoin/myriadcoin,myriadcoin/myriadcoin,GroestlCoin/GroestlCoin,gjhiggins/vcoincore,viacoin/viacoin,GroestlCoin/bitcoin,GroestlCoin/bitcoin,gjhiggins/vcoincore,1185/starwels,starwels/starwels,viacoin/viacoin,starwels/starwels,starwels/starwels,peercoin/peercoin,1185/starwels,peercoin/peercoin,myriadteam/myriadcoin,myriadcoin/myriadcoin,peercoin/peercoin,ppcoin/ppcoin,starwels/starwels,GroestlCoin/GroestlCoin,DigitalPandacoin/pandacoin,viacoin/viacoin,myriadteam/myriadcoin,gjhiggins/vcoincore,ppcoin/ppcoin,DigitalPandacoin/pandacoin,peercoin/peercoin,starwels/starwels,peercoin/peercoin,GroestlCoin/GroestlCoin,starwels/starwels,myriadcoin/myriadcoin,1185/starwels,myriadteam/myriadcoin,gjhiggins/vcoincore,DigitalPandacoin/pandacoin,ppcoin/ppcoin,GroestlCoin/GroestlCoin