Fixes #9133: Require Python 3.8+ to run upgrade.sh

[markkuleinio]

Fixes: #9133

Updates the Python version checks in upgrade.sh to require 3.8+ (as required in NetBox 3.2).

# ./upgrade.sh
--------------------------------------------------------------------
ERROR: Unsupported Python version: Python 3.7.3. NetBox requires
Python 3.8 or later. To specify an alternate Python executable, set
the PYTHON environment variable. For example:

  sudo PYTHON=/usr/bin/python3.8 ./upgrade.sh

To show your current Python version: python3 -V
--------------------------------------------------------------------
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