Your phone will run iOS 7, but it requires a computer to boot up every time it restarts.
Downgrading your iPhone to a previous iOS version may void your warranty and may not be supported by Apple. Proceed at your own risk.
The current gold standard for downgrading and dual-booting older A-series devices.
To attempt this, you generally need a (macOS is much more reliable for these exploits than Windows or Linux) and the following software: