Consolidate how deployCesium works in gulpfile.cjs

`gulpfile.cjs` in master will currently upload all files to a single upload location: https://github.com/CesiumGS/cesium/blob/70478525bd196b7cb30bd3669dfc51082adca6ee/gulpfile.cjs#L455

`gulpfile.cjs` in the `cesium.com` branch however needs to upload different directories to different upload locations, because Sandcastle lives at `sandcastle.cesium.com`, whereas the docs live at `https://cesium.com/docs/cesiumjs-ref-doc/`: https://github.com/CesiumGS/cesium/blob/0d17b127f550107e396e3806ed0253f9a3a9145d/gulpfile.cjs#L466

This difference made it hard to resolve a merge conflict during release, see discussion in: https://github.com/CesiumGS/cesium/pull/8715

We should consolidate how the gulpfile deploys Cesium to avoid conflicts like this.

Start in gulpfile.cjs at the deployCesium implementation around the referenced lines in master and the cesium.com branch. Read the discussion in pull request #8715 to understand the merge conflict and compare the required upload destinations for Sandcastle and the documentation. Done means the deployment behavior is consolidated so releases avoid this branch-specific conflict while preserving both destinations.

Written by the indexing model from the issue text.