[FIX] Replace python with python3 in install scripts, docs and shebangs

[x15sr71]

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Updated install scripts, documentation and shebangs to use python3 consistently, as the python command is not reliable across different system configurations

Changes:

• install/install.sh — replaced python with python3 in all command invocations, updated update-alternatives to reference python3 instead of python3.9
• install/installation.md — updated cron setup instruction and gunicorn bootstrap command to use python3
• README.md — updated manage.py usage example to use python3
• mod_ci/cron.py, install/init_db.py, install/sample_db.py, mod_upload/progress_ftp_upload.py — updated shebangs from #!/usr/bin/python to #!/usr/bin/python3

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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