Update ci.yml from macos-10.15 to macos-11. (#2940)

Author: jimblandy

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
 
           # MacOS
           - name: MacOS x86_64
-            os: macos-10.15
+            os: macos-11
             target: x86_64-apple-darwin
             tool: clippy
             # Mac has no software runners, so don't run tests
@@ -54,7 +54,7 @@ jobs:
 
           # IOS
           - name: IOS aarch64
-            os: macos-10.15
+            os: macos-11
             target: aarch64-apple-ios
             tool: clippy
             kind: other