macos-latest updated in aug to be macos-15 and now was seeing kex test failure, revert to macos-14

.github/workflows/singlethread-check.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest, macos-latest ]
+        os: [ ubuntu-latest, macos-14 ]
     name: Build wolfssl
     runs-on: ${{ matrix.os }}
     timeout-minutes: 4
@@ -46,7 +46,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest, macos-latest ]
+        os: [ ubuntu-latest, macos-14 ]
         config: [
           '',
           '--enable-all',