Change Mac runtime to MacOS.

Author: raphw

.github/workflows/main.yml

@@ -25,7 +25,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-18.04, macOS-10.14, windows-2019]
+        os: [ubuntu-18.04, macOS-10.15, windows-2019]
         java: [8, 11, 13]
         architecture: [x32, x64]
     runs-on: ${{ matrix.os }}