Baker: Notice when a delegate has no slots at current level

What

Add a notice-level log with delegates without slots at current level.

Why

As mentioned in !15846 (comment 2245049660), current log that no shard as been assigned to DAL baker is only displayed for member of the Tenderbake committee. To identify easily, only while reading logs, if one has shard assigned, it is required to combine the information from !15846 (merged) with the one from this MR.

How

Add a notice-level event triggered when the state_level is updated to the new level.

Manually testing the MR

Following Eugen's comment on testing it in the sandbox, you should observe:

Dec 12 10:32:31.007: received new proposal BKz6u6pqMUFWH8GW1G2Xx3zJmFY3bYg6hPCkcfhJLaHYwoPiycq at
Dec 12 10:32:31.007:   level 717, round 0
Dec 12 10:32:31.010: The following delegates have no attesting rights at level 717:
Dec 12 10:32:31.010:   activator (tz1TGu6TN5GSez2ndXXeDX6LgUDvLzPLqgYV)
Dec 12 10:32:31.013: received new forge event:
Dec 12 10:32:31.013:   preattestation ready for delegate bootstrap1 (tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx) at level 717 (round 0)

The two middle lines are the newly added logs.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR