Revert "Install Python 3.9 on CentOS/Rocky 8.5 machines"

Summary:
Per T1706, Python 3.6 was previously shipped with CentOS 8.5.

The situation evolved, and versions currently available are:

• CentOS Stream 10: 3.12 by default
• RockyLinux 9.4: 3.9 by default, 3.11 available

This reverts commit e509029a471257f4c987f2a6b2680f2b56d3a9df.

Ref T1982

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1982

Differential Revision: https://devcentral.nasqueron.org/D3371