CI: Build on MacOS 11 intel, not EOL 10.15

[clintoncwolfe]

Description

On our MacOS Intel pipeline, drop support for EOL MacOS 10.15, and instead build on MacOS 11.

Related Issue

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have run the pre-merge tests locally and they pass.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• If Gemfile.lock has changed, I have used --conservative to do it and included the full output in the Description above.
• All new and existing tests passed.
• All commits have been signed-off for the Developer Certificate of Origin.

[netlify]

👷 Deploy Preview for chef-workstation processing.

Name	Link
🔨 Latest commit	baf8390
🔍 Latest deploy log	https://app.netlify.com/sites/chef-workstation/deploys/6526bfce5d0d6a0008af9621

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[sonarqube-for-infrastructure-prod]

SonarQube Quality Gate

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[sean-simmons-progress]

before merging ill let you all know when the latest buildkite-platform release is out then we can proceed.