CI: Release message to internal slack

[MarieSacksick]

Describe the bug

Since early december, the CI part in charge of releasing is not functional anymore.

Steps/Code to Reproduce

Do a release and check the logs (for instance for skore 0.6.1.).

Expected Behavior

We should have a message in our internal slack.

Actual Behavior

Nothing happens, that part of the CI fails (but it doesn't block anything, and this behavior is correct, we don't want a release to be blocked because of slack).

Environment

N/A

[MarieSacksick]

Typically the kind of issues we could tackle inbetween functional developments.