DevOps Azure will not support macOS 10.14 as of December 10

[Vadim-Valdis Yudaev]

Hi here,

Since that DevOps Azure will not support macOS 10.14 as of December 10, it's
worth considering bump the version to 10.15. Otherwise, we may get a broken
check again for PRs.

> The macOS-10.14 environment is deprecated and will be removed on December 10,
> 2021. For more details see
> https://devblogs.microsoft.com/devops/hosted-pipelines-image-deprecation/