Simplify concept about if-statements

[sanderploegsma]

The existing conditionals-if concept was covering way too much: next to introducing if statements it also covered switch statements and even mentioned ternary operators. Both of these have their own concepts, so the scope had to be reduced.

The accompanying concept exercise blackjack was also way too complicated at this stage of the learning journey. I had a look at other tracks and saw that C# uses the cars-assemble exercise to introduce concepts for numbers and if-else statements, which makes way more sense. I therefore deprecated the blackjack exercise and added the simplified and renamed concept if-else-statements to the cars-assemble exercise instead.

Fixes #2365
Fixes #2311

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