Scale librarian book trade with librarian level

Librarians should only give low level enchanted books at low levels, and higher level versions of those books when at higher levels.

For example, say you have an apprentice librarian that gives "fortune I". At a low level they wouldn't be able give you higher than that. After leveling them up a couple time you can get "fortune II" or "fortune III", but you wouldn't get something like "Smite V" as that wouldn't be this librarians expertise.

This fixes 2 problems.

1) Cycling librarian trades is boring. Librarians giving only low level books at apprentice level would decrease the number of possibilities, making it easier to get the right category of enchantment. This would decrease time spent mindlessly grinding for the right enchantment.

2) Librarian enchanted books are overpowered early game. By having librarians give higher level books of their earlier trades after getting leveled up, you push access to better enchantments closer to the late game instead of just requiring boring grind to "get lucky". It's much harder to level up the librarians early game, so early on players wouldn't be able to easily access the best enchantments.

As an extra bonus, this would thematically fit the librarian "learning" and becoming better at their specific niche.