ci(docs-infra): rename 'upload-server' to 'preview-server'

The server no longer has files uploaded to it. Instead it is more
accurate to refer to it as dealing with "previews" of PRs.

aio/aio-builds-setup/dockerbuild/scripts-sh/preview-server-test.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -eu -o pipefail
+
+# Start the preview-server instance
+appName=aio-preview-server-test
+if [[ "$1" == "stop" ]]; then
+  pm2 delete $appName
+else
+  source aio-test-env
+  pm2 start $AIO_SCRIPTS_JS_DIR/dist/lib/verify-setup/start-test-preview-server.js \
+    --uid $AIO_WWW_USER \
+    --log /var/log/aio/preview-server-test.log \
+    --name $appName \
+    --no-autorestart \
+    ${@:2}
+fi