Update doc to drop support Python 3.8 when it meets its the end of life

[mingxin-zheng]

Describe the bug

Since Python 3.8 is near end of life, and MONAI start dropping tests. Please also update the installation documentation to reflect this change.

• https://github.com/Project-MONAI/MONAI/blob/dev/docs/images/python.svg
• https://github.com/Project-MONAI/MONAI/blob/dev/CONTRIBUTING.md
• https://github.com/Project-MONAI/MONAI/blob/dev/docs/source/installation.md

• MONAI/monai/apps/auto3dseg/auto_runner.py

  Line 554 in 9554f47

  - single GPU/CPU or multinode bcprun: "python " or "/opt/conda/bin/python3.8 ",

• MONAI/monai/auto3dseg/utils.py

  Line 410 in 9554f47

  cmd_prefix: the command prefix to run the script, e.g., "python", "python -m", "python3", "/opt/conda/bin/python3.8 ".

• MONAI/setup.cfg

  Line 24 in 9554f47

  Programming Language :: Python :: 3.8

• https://github.com/Project-MONAI/MONAI/blob/dev/CONTRIBUTING.md

[KumoLiu]

It's already fixed in #7909.

[mingxin-zheng]

Thanks!