Bump Azure CI Mac image to macos-11

[ricrogz]

According to https://learn.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops&tabs=yaml#recent-updates, the image we were using, macos-10.15, is no longer supporter, and won't run, as in https://github.com/schrodinger/coordgenlibs/runs/16259208536 (CI run for https://github.com/schrodinger/coordgenlibs/pull/125)