Bump up minimum Python version

[gbanasiak]

The minimum Python version (MIN_PY_VER) is not aligned with 3.8.x version (PY38) which breaks installation instructions for development with the following symptoms:

grzegorz:src/elastic% pyenv versions
* system (set by /Users/grzegorz/.pyenv/version)
  3.8.16
  3.9.16
  3.10.10
  3.11.0
  3.11.3

grzegorz:src/elastic% git clone https://github.com/elastic/rally.git && cd rally
Cloning into 'rally'...
remote: Enumerating objects: 16834, done.
remote: Counting objects: 100% (315/315), done.
remote: Compressing objects: 100% (156/156), done.
remote: Total 16834 (delta 167), reused 275 (delta 151), pack-reused 16519
Receiving objects: 100% (16834/16834), 9.31 MiB | 8.24 MiB/s, done.
Resolving deltas: 100% (12782/12782), done.

grzegorz:elastic/rally% make prereq
pyenv install --skip-existing 3.8.16
pyenv install --skip-existing 3.9.16
pyenv install --skip-existing 3.10.10
pyenv install --skip-existing 3.11.0
pyenv local 3.8.16
IMPORTANT: please type pyenv init, follow the instructions there and restart your terminal before proceeding any further.

grzegorz:elastic/rally% pyenv versions
pyenv: version `3.8.13' is not installed (set by /Users/grzegorz/Documents/src/elastic/rally/.python-version)
  system
* 3.8.16 (set by /Users/grzegorz/Documents/src/elastic/rally/.python-version)
* 3.9.16 (set by /Users/grzegorz/Documents/src/elastic/rally/.python-version)
* 3.10.10 (set by /Users/grzegorz/Documents/src/elastic/rally/.python-version)
* 3.11.0 (set by /Users/grzegorz/Documents/src/elastic/rally/.python-version)
  3.11.3

grzegorz:elastic/rally% cat .python-version 
3.8.16
3.9.16
3.10.10
3.11.0
3.8.13 <--- HERE

grzegorz:elastic/rally% jq '.python_versions' .ci/variables.json 
{
  "PY38": "3.8.16",
  "PY39": "3.9.16",
  "PY310": "3.10.10",
  "PY311": "3.11.0",
  "MIN_PY_VER": "3.8.13" <--- HERE
}

grzegorz:elastic/rally% make install
pyenv: version `3.8.13' is not installed (set by /Users/grzegorz/Documents/src/elastic/rally/.python-version)
Created python3 venv under /Users/grzegorz/Documents/src/elastic/rally/.venv.
. .venv/bin/activate; python3 -m pip install --upgrade hatch==1.3.1 hatchling==1.6.0 pip==22.2 wheel==0.37.1
/bin/bash: .venv/bin/activate: No such file or directory
pyenv: version `3.8.13' is not installed (set by /Users/grzegorz/Documents/src/elastic/rally/.python-version)
make: *** [install-user] Error 1

This PR increases the minimum Python version from 3.8.13 to 3.8.16 restoring the alignment.

[pquentin]

Thanks! LGTM