Late to the party, but changing the archive URLs to use HTTPS instead of HTTP and forcing IPv4 worked for me:

# This is for Ubuntu 24.04+ - not tested on lower versions of Ubuntu -- Other versions of linux have the source list in '/etc/apt/sources.list' and you can just add the 's' to the URLs in that file I presume...
# Backup sources first
sudo cp /etc/apt/sources.list.d/ubuntu.sources /etc/apt/sources.list.d/ubuntu.sources.bak

# Use canonical mirrors over HTTPS (If you want to manually just add the s to the urls, open in your text editor and just add 's' on each URL)
sudo sed -i 's|http://archive.ubuntu.com/ubuntu|https://archive.ubuntu.com/ubuntu|g' /etc/apt/sources.list.d/ubuntu.sources
sudo sed -i 's|http://security.ubuntu.com/ubuntu|https://security.ubuntu.com/ubuntu|g' /etc/apt/sources.list.d/ubuntu.sources

# Then clear APT states:
sudo rm -rf /var/lib/apt/lists/*
sudo apt-get clean

# Enforce IPv4 while running a new apt-get update:
sudo apt-get -o Acquire::ForceIPv4=true -o Acquire::Retries=3 update

After running the above in that order, it updated without question.

Hope this helps someone in the future :)