Azure Pipeline: switch to the latest agent pools

Azure Pipeline: switch to the latest agent pools

It would seem that at least the `vs2015-win2012r2` pool (which we use
via its old name, `Hosted`) is about to be phased out. Let's switch
before that.

While at it, use the newer pool names as suggested at
https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops#use-a-microsoft-hosted-agent

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>