Change installer doc from EOL python 3.8 to 3.9, remove pyenv recommendation

[jamshid]

NBD but might as well change https://esrally.readthedocs.io/en/stable/install.html mention of "Python 3.8 or better" to "Python 3.9 or better" since 3.8 recently EOL'd and python3.9 is a simple package install on RHEL/RockyLinux 8+.

FWIW I had to also install python39-devel and "Development Tools" to avoid e.g. an error with creating build/temp.linux-aarch64-cpython-39/psutil but maybe that's because I'm running on an arm64 machine.

% docker run -ti rockylinux:8 bash # to test install on a bare bones server
yum install -y python39-pip python39-devel 
yum group install -y "Development Tools"
pip3.9 install esrally
esrally --help
esrally race --track=nyc_taxis --test-mode --pipeline=benchmark-only --target-hosts=ELASTICSEARCH:9200

I suggest removing pyenv, or add "if you need multiple python versions", because it's a nightmare to install but maybe I'm missing something. Fortunately any modern linux should support a python that works with esrally and hopefully esrally remains compatible with new python versions.