Update CI workflow for Julia and checkout action (#659)

Updated CI configuration to use actions/checkout@v6 and added
macOS-15-intel for macos x86 tests.

Author: ViralBShah

.github/workflows/ci.yml

@@ -30,11 +30,11 @@ jobs:
           - os: macOS-latest
             julia-arch: aarch64
             julia-version: 'nightly'
-          - os: macOS-13
+          - os: macOS-15-intel
             julia-arch: x64
             julia-version: 'nightly'
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v2
         with:
           version: ${{ matrix.julia-version }}
@@ -62,7 +62,7 @@ jobs:
         julia-arch:
           - x64
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v2
         with:
           version: ${{ matrix.julia-version }}
@@ -75,7 +75,7 @@ jobs:
   docs:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v2
         with:
           version: 'nightly'