ci: bump actions/upload-artifact from 4 to 6 (#34)

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] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -171,7 +171,7 @@ jobs:
           name: codecov-umbrella
 
       - name: Archive coverage report
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: coverage-report
           path: lcov.info

.github/workflows/release.yml

@@ -107,7 +107,7 @@ jobs:
           7z a ../../../mcp-execution-${{ matrix.asset_suffix }}.zip ${{ matrix.server_artifact }}
 
       - name: Upload CLI artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: mcp-execution-cli-${{ matrix.asset_suffix }}
           path: |
@@ -116,7 +116,7 @@ jobs:
           retention-days: 7
 
       - name: Upload MCP Server artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: mcp-execution-${{ matrix.asset_suffix }}
           path: |