All build legs need to move off macOS 10.15

[hoyosjs]

There's currently a brownout and the image is scheduled to be decommissioned by 9.30; we need to patch all pertinent servicing branches.

• Main
• 7.0
• 6.0
• 3.1 (corefx + coreclr + core-setup)

cc: @dotnet/runtime-infra

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

Tagging subscribers to this area: @dotnet/runtime-infrastructure
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Issue Details

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There's currently a brownout and the image is scheduled to be decommissioned by 9.30; we need to patch all pertinent servicing branches.

cc: @dotnet/runtime-infra

Author:	hoyosjs
Assignees:	-
Labels:	blocking, area-Infrastructure
Milestone:	-

[steveisok]

#75294 is for 6.0

[akoeplinger]

This is done.