Prepare upload scripts for Python 3.7 removal (#10092)

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: #10092

ci/cpu/prebuild.sh

@@ -4,12 +4,13 @@
 set -e
 
 DEFAULT_CUDA_VER="11.5"
+DEFAULT_PYTHON_VER="3.8"
 
 #Always upload cudf Python package
 export UPLOAD_CUDF=1
 
 #Upload libcudf once per CUDA
-if [[ "$PYTHON" == "3.7" ]]; then
+if [[ "$PYTHON" == "${DEFAULT_PYTHON_VER}" ]]; then
     export UPLOAD_LIBCUDF=1
 else
     export UPLOAD_LIBCUDF=0
@@ -23,7 +24,7 @@ else
 fi
 
 #We only want to upload libcudf_kafka once per python/CUDA combo
-if [[ "$PYTHON" == "3.7" ]] && [[ "$CUDA" == "${DEFAULT_CUDA_VER}" ]]; then
+if [[ "$PYTHON" == "${DEFAULT_PYTHON_VER}" ]] && [[ "$CUDA" == "${DEFAULT_CUDA_VER}" ]]; then
     export UPLOAD_LIBCUDF_KAFKA=1
 else
     export UPLOAD_LIBCUDF_KAFKA=0