Remove Mac OS 11 environment from target as it is phased out

System message “a brownout will take place on June, 8:00 AM – 2:00 PM
EST to raise awareness of the upcoming macOS-11 environment removal.”
so we need to remove Mac OS 11 environment.

.github/workflows/continuous-integration.yaml

@@ -11,7 +11,7 @@ jobs:
   run-testsuite:
     strategy:
       matrix:
-        os: ['ubuntu-latest', 'macos-11', 'macos-12', 'macos-13', 'macos-14']
+        os: ['ubuntu-latest', 'macos-12', 'macos-13', 'macos-14']
     runs-on: '${{ matrix.os }}'
     name: 'Run Testsuite'
     steps:
@@ -22,7 +22,7 @@ jobs:
   install-macports:
     strategy:
       matrix:
-        os: ['macos-11', 'macos-12', 'macos-13', 'macos-14']
+        os: ['macos-12', 'macos-13', 'macos-14']
         version: ['2.9.3']
         prefix: ['/opt/local']
     runs-on: '${{ matrix.os }}'