[CI]: Bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/build-vfx-reference-platform.yml

@@ -45,7 +45,7 @@ jobs:
         cmake --install build
 
     - name: Upload archive
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: openassetio-CY2024
         path: openassetio-checkout/build/dist

.github/workflows/build-wheels.yml

@@ -80,7 +80,7 @@ jobs:
           # Ensure .pyi stub tests won't be skipped
           OPENASSETIO_TEST_ENABLE_PYTHON_STUBGEN: 1
 
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         with:
           name: "openassetio-wheels-${{ matrix.config.os }}-${{ matrix.python-build }}"
           path: ./wheelhouse/*.whl

.github/workflows/build.yml

@@ -61,7 +61,7 @@ jobs:
         cmake --install build
 
     - name: Upload archive
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: openassetio-${{ matrix.config.os }}
         path: |
@@ -99,7 +99,7 @@ jobs:
         ! grep -qE "^.*?/src/[^:]+:[0-9]+: ?[a-zA-Z]+: ?.*$" doxygen-log.txt
 
     - name: Expose archive docs artifact
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: doxygen-documentation
         path: doc/doxygen/html