No update, but you can keep your eye on the official packaging guide, when this gets updated it means the build process has been updated to include the new distro version. Thanks!
Getting the same issue when trying to install Terraform on Docker image based from Ubuntu 23.10.
I also tried the way below to skip checking the Codename (mantic) and directly download the ZIP package of Terraform. Every command looks running well without any error reported.
Once this distribution is supported its Terraform packages will be exactly the same as for every other Debian and Ubuntu distribution release, because Terraform is distributed as a statically-linked executable that depends only on the Linux kernel and not on anything else in your distribution.
Therefore you can use the name of any already-supported release instead of mantic and get the same effect.
This is not necessarily true for all other packages in the HashiCorp APT repository, because some of the software distributed there depends on other system libraries. Therefore I would not suggest using this workaround if you intend to install anything other than the terraform package, but if Terraform is all you need then any supported distribution codename should work just the same.