📜 Changes the slides and workbooks to fit the new level structure

[MarleenGilsing]

@LucasPeters010 could you make the following changes in all languages? I can elaborate in the meeting tomorrow.

For the slides:
Level 0 through 4 stay the same
Level 5 stays the same, but has slide 6 ('another new command') removed
Level 6 becomes 7
Level 7 becomes 8
Level 8 becomes 9
Level 9 becomes 10
Level 10 becomes 11
Level 11 is thrown away
Level 12 stays the same, but has slide 2 and 3 thrown away
Level 13 through 18 are thrown away

Level 6 and 13-16 will later get new content, that will fit the new level structure.

For the workbooks:
Level 1 through 5 stay the same
Level 6 becomes 7
Level 7 becomes 8
Level 8 becomes 9
Level 9 becomes 10