Add ssh key check (#6149)

ci/check-ssh-keys.sh

@@ -0,0 +1,4 @@
+if grep -q rsa net/scripts/solana-user-authorized_keys.sh; then
+   echo "No rsa keys allowed, small key sizes are insecure."
+   exit 1
+fi

ci/test-checks.sh

@@ -24,6 +24,7 @@ _ ci/nits.sh
 _ ci/order-crates-for-publishing.py
 _ book/build.sh
 _ book/build-svg.sh
+_ ci/check-ssh-keys.sh
 
 {
   cd programs/bpf