TL;DR

What if your OpenSSH ssh is before release-7.3 and you cannot use ProxyJump? You can try with ProxyCommand!

As observed by crimson-egret:

ProxyJump was added in OpenSSH 7.3 but is nothing more than a shorthand for using ProxyCommand

Hence, the example from ProxyJump can be adapted to work back up to (and including) release-5.4, like this:

Host jumphost
   HostName jumphost.local
   User bar
   IdentityFile ~/.ssh/jumphost.key
Host target
   HostName target.internal
   User galook
   IdentityFile ~/.ssh/target.key
   ProxyCommand ssh -W %h:%p jumphost

See also: Old Methods of Passing Through Jump Hosts.