Bump MAC OS Version to currently supported version azure

[hmaarrfk]

Seems like the old image isn't supported anymore.

https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops&tabs=yaml

[hmaarrfk]

Same issue as #469.

Closing.

[hmaarrfk]

The other PR is about cron jobs, while this is about bumping the OS image.

[hmaarrfk]

Conda forge is using the settings:

Agent name: 'Azure Pipelines 20'
Agent machine name: 'Mac-1612759872240'
Current agent version: '2.181.1'
Operating System
Mac OS X
10.15.7
19H114
Virtual Environment
Environment: macos-10.15
Version: 20210130.1
Included Software: https://github.com/actions/virtual-environments/blob/macOS-10.15/20210130.1/images/macos/macos-10.15-Readme.md
Current image version: '20210130.1'
Agent running as: 'runner'
Prepare build directory.
Set build variables.
Download all required tasks.
Downloading task: CmdLine (2.182.0)
Checking job knob settings.
   Knob: AgentToolsDirectory = /Users/runner/hostedtoolcache Source: ${AGENT_TOOLSDIRECTORY} 
Finished checking job knob settings.
Plugin: 'Test Result Parser plugin' is running in background.
Plugin: 'Test File Publisher plugin' is running in background.
Start tracking orphan processes.