fix(ci): use macos-15 runner for full C++20 support in Z3 build

Apple Clang on macos-14 (Xcode 15) lacks full P0960R3 support
(parenthesized aggregate initialization). Switch to macos-15 which
ships Xcode 16 / Apple Clang 16 with complete C++20 support.

Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>
Signed-off-by: Paolo Dettori <dettori@us.ibm.com>

Author: pdettori

.github/workflows/kagenti-release-cli.yml

@@ -108,7 +108,7 @@ jobs:
 
   build-cli-macos:
     name: Build CLI (macOS arm64)
-    runs-on: macos-14
+    runs-on: macos-15
     timeout-minutes: 60
     steps:
       - name: Checkout
@@ -134,7 +134,6 @@ jobs:
 
       - name: Build openshell-cli
         env:
-          CXXFLAGS: "-std=c++20"
           READ_ONLY_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: cargo build --release -p openshell-cli --features bundled-z3