Getting error while installing using APT repository.

Hello,
I'm trying to install GoCD on Ubuntu using the APT repository following the instructions [here](https://docs.gocd.org/current/installation/install/server/linux.html#debian-based-distributions-ie-ubuntu)
However, I'm getting the following error
```
sudo apt-get update
deb [signed-by=/etc/apt/keyrings/gocd.gpg] https://download.gocd.org /
Hit:1 https://dl.google.com/linux/chrome/deb stable InRelease
Get:2 https://download.gocd.org InRelease [2478 B]
Err:2 https://download.gocd.org InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D8843F288816C449
Hit:3 http://security.ubuntu.com/ubuntu jammy-security InRelease
Hit:4 http://in.archive.ubuntu.com/ubuntu jammy InRelease
Hit:5 http://in.archive.ubuntu.com/ubuntu jammy-updates InRelease
Hit:6 http://in.archive.ubuntu.com/ubuntu jammy-backports InRelease
Reading package lists... Done
W: GPG error: https://download.gocd.org InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D8843F288816C449
E: The repository 'https://download.gocd.org InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

```