2480decbc7
before this change, when user follows the instruction, they'd get
```console
$ sudo apt-get update
Hit:1 http://us-east-1.ec2.archive.ubuntu.com/ubuntu noble InRelease
Hit:2 http://us-east-1.ec2.archive.ubuntu.com/ubuntu noble-updates InRelease
Hit:3 http://us-east-1.ec2.archive.ubuntu.com/ubuntu noble-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu noble-security InRelease
Get:5 https://downloads.scylladb.com/downloads/scylla/deb/debian-ubuntu/scylladb-6.2 stable InRelease [7550 B]
Err:5 https://downloads.scylladb.com/downloads/scylla/deb/debian-ubuntu/scylladb-6.2 stable InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A43E06657BAC99E3
Reading package lists... Done
W: GPG error: https://downloads.scylladb.com/downloads/scylla/deb/debian-ubuntu/scylladb-6.2 stable InRelease: The following signatures couldn't be verified because the public key is not av
ailable: NO_PUBKEY A43E06657BAC99E3
E: The repository 'https://downloads.scylladb.com/downloads/scylla/deb/debian-ubuntu/scylladb-6.2 stable 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.
```
because the packages were signed with a different keyring.
in this change, we import the new pubkey, so that the pacakge manager
can
verify the new packages (2024.2+ and 6.2+) signed with the new key.
see also https://github.com/scylladb/scylla-ansible-roles/issues/399
and https://forum.scylladb.com/t/release-scylla-manager-3-3-1/2516
for the annonucement on using the new key.
Fixes scylladb/scylladb#21557
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes scylladb/scylladb#21524
(cherry picked from commit 1cedc45c35)
Closes scylladb/scylladb#21588
