Bug 1717108 - Switch macOS startup tests from 10.14 to 10.15 r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D118232

taskcluster/ci/startup-test/kind.yml

@@ -84,7 +84,7 @@ jobs:
             upstream_kind: repackage
             upstream_artifact: target.dmg
             binary: Contents/MacOS/firefox
-        worker-type: t-osx-1014
+        worker-type: t-osx-1015-r8
         shipping-product: firefox
         attributes:
             build_platform: macosx64-shippable