$ ssh -V OpenSSH_8.2p1 Ubuntu-4, OpenSSL 1.1.1f 31 Mar 2020
$ snap run --shell git-ubuntu $ which ssh /snap/core/current/usr/bin/ssh $ ssh -V OpenSSH_7.2p2 Ubuntu-4ubuntu2.8, OpenSSL 1.0.2n 7 Dec 2017
So you are using ssh from the core snap, instead of the host, with host ssh configs.
Either you need to prefer ssh from the host (or all of PATH?), or upgrade git-ubuntu to use "base: core20".
$ ssh -V
OpenSSH_8.2p1 Ubuntu-4, OpenSSL 1.1.1f 31 Mar 2020
$ snap run --shell git-ubuntu current/ usr/bin/ ssh
$ which ssh
/snap/core/
$ ssh -V
OpenSSH_7.2p2 Ubuntu-4ubuntu2.8, OpenSSL 1.0.2n 7 Dec 2017
So you are using ssh from the core snap, instead of the host, with host ssh configs.
Either you need to prefer ssh from the host (or all of PATH?), or upgrade git-ubuntu to use "base: core20".