The oldest version I can deploy from on Xcode is 8.
How can I make my app compatible with older ios versions?
You can still use iOS 7.x as a deployment target in the store, but not w/current tools (Xcode 8.3.2). I'd wonder about doing so, however, as number of users on an iOS that far back is in low single digits.
Note that according to iTunes Connect, minimum now for the store is Xcode 6, which supports iOS 4.3 or iOS 5.1.1 minimum deployment, so to reach your goal of supporting older iOSs, you'd need to use older Xcode - restrictions apply, mainly not being able to also support newer iOSs, so...