Use macos-11 runner

Author: termoshtt

.github/workflows/openblas-src.yml

@@ -61,7 +61,7 @@ jobs:
       if: ${{ matrix.triple == 'x64-windows-static' }}
 
   macos:
-    runs-on: macos-10.15
+    runs-on: macos-11
     strategy:
       fail-fast: false
       matrix: