Prepare upload scripts for Python 3.7 removal (rapidsai#4500)

As we will remove Python 3.7, we need to update the Python version in the upload scripts

Authors:
  - Jordan Jacobelli (https://github.com/Ethyling)

Approvers:
  - Sevag Hanssian (https://github.com/sevagh)
  - AJ Schmidt (https://github.com/ajschmidt8)

URL: rapidsai#4500

ci/cpu/prebuild.sh

@@ -1,8 +1,9 @@
 #!/usr/bin/env bash
+# Copyright (c) 2022, NVIDIA CORPORATION.
 
 export UPLOAD_CUML=1
 
-if [[ "$PYTHON" == "3.7" ]]; then
+if [[ "$PYTHON" == "3.8" ]]; then
     export UPLOAD_LIBCUML=1
 else
     export UPLOAD_LIBCUML=0