ci: Update macOS version to 11 from unsupported 10.14

It doesn't look like there are any provided macOS 10.14 runners left so
CI never fully completes. 10.15 is also being depreciated on 5/31/2022
(in two days at the time of writing), so there is little reason to
update to that.

macOS 10.15 depreciation issue:
actions/runner-images#5583

.github/workflows/macos.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-10.14]
+        os: [macos-11]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v1