ubuntu 20.04.1 LTS: No verification for DSA (only for RSA)

I've just downloaded the iso file from https://releases.ubuntu.com/jammy/ and also SHA256SUMS and SHA256SUMS.gpg. When performing, as described in https://ubuntu.com/tutorials/how-to-verify-ubuntu#1-overview ,

gpg --keyid-format long --verify SHA256SUMS.gpg SHA256SUMS

I get

gpg: Signature made Thu Aug 11 13:07:33 2022 CEST
gpg:                using RSA key 843938DF228D22F7B3742BC0D94AA3F0EFE21092
gpg: Good signature from "Ubuntu CD Image Automatic Signing Key (2012)    
<cdimage@ubuntu.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 8439 38DF 228D 22F7 B374  2BC0 D94A A3F0 EFE2 1092

The second good signature is missing. What does this mean?

Information: The command

gpg --list-keys --with-fingerprint 0xFBB75451 0xEFE21092

gives

pub   rsa4096 2012-05-11 [SC]
  8439 38DF 228D 22F7 B374  2BC0 D94A A3F0 EFE2 1092
uid           [ unknown] Ubuntu CD Image Automatic Signing Key (2012)   
<cdimage@ubuntu.com>

pub   dsa1024 2004-12-30 [SC]
  C598 6B4F 1257 FFA8 6632  CBA7 4618 1433 FBB7 5451
uid           [ unknown] Ubuntu CD Image Automatic Signing Key <cdimage@ubuntu.com>

When performing

echo "c396e956a9f52c418397867d1ea5c0cf1a99a49dcf648b086d2fb762330cc88d *ubuntu-22.04.1-desktop-amd64.iso" | shasum -a 256 --check

I get (after ca. 1 minute)

ubuntu-22.04.1-desktop-amd64.iso: OK

so everything may be correct. NB. In my opinion the tutorial https://ubuntu.com/tutorials/how-to-verify-ubuntu#1-overview is a bit misleading.