The validity of the signature key has expired, and only after a few days Microsoft generated a new key. The key replacement in your system is not automatic and you need to delete the old key and add the new one.
There are two ways two solve the issue: one, using the traditional apt-key add command, the other one adding manually the new key to the keyring.
# Solution valid until Ubuntu 21.04 (including it)
You can delete the no longer valid key:
sudo apt-key del 1F3045A5DF7587C3
and then re-add the new valid key:
curl https://repo.skype.com/data/SKYPE-GPG-KEY | sudo apt-key add -
Running sudo apt update && sudo apt upgrade you should see no errors.
# Solution valid in Ubuntu 21.04 and future releases
When you try to add an APT repository key using apt-key in Ubuntu, you may see the following message:
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
The apt-key man page mentions that:
use of apt-key is deprecated, except for the use of apt-key del in maintainer scripts to remove existing keys from the main keyring.
So, if you use a version of Ubuntu until 21.04, you are ok using apt-key del and apt-key add, but for the following versions you must manually add the key in the keyring (in Ubuntu 21.04 both solutions work perfectly: I tested both of them).
You can delete the no longer valid key:
sudo apt-key del 1F3045A5DF7587C3
Download the key and add it to the keyring:
curl https://repo.skype.com/data/SKYPE-GPG-KEY | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/skype-stable-archive-keyring.gpg
Open the skype-stable.list file...
sudo nano /etc/apt/sources.list.d/skype-stable.list
...and modify the first line in this way:
deb [arch=amd64 signed-by=/etc/apt/trusted.gpg.d/skype-stable-archive-keyring.gpg] https://repo.skype.com/deb stable main
Running sudo apt update && sudo apt upgrade you should see no errors.
