Proposal to push junction refs from Z11 to perhaps Z13

Hello, I realize this is a very mature project and changes aren't made willy nilly. But I wanted to suggest a change that has been on my mind for many years actually. I think that highway exit numbers (the highway_junction ref) are rendered too early in the zoom. This really depends on the city that you look at. For example, London has essentially none of these, no problem there. Rome only has these on the A90 ring and further out - looks fine either way. When I look at European cities in general, I see minimal motorway junctions. Here are some examples. I copied these comparisons from the last time this rendering was adjusted in #3482 <details> <summary><b>Comparison of Warsaw, Malmö, Amsterdam at Z 12</b> (small improvement)</summary> `#12/52.2322/20.9537`  `#12/55.5840/12.9635` <img width="1668" height="952" alt="Image" src="https://github.com/user-attachments/assets/1c750076-b6de-4e34-848d-9a07491c0458" /> `#12/52.3685/4.9116`  </details> <details> <summary><b>Comparison of Warsaw, Malmö, Amsterdam at Z 11</b> (small improvement)</summary> `#11/52.2320/20.9537`  `#11/55.5838/12.9639` <img width="1668" height="952" alt="Image" src="https://github.com/user-attachments/assets/14043c7c-9fac-4d7b-af6a-7139ad7c4a67" /> `#11/52.3685/4.9116`  </details> I think this is a small improvement to less car dependent cities like those. I'm suggesting this change because I think it's a large improvement to American cities that have motorways with tagged (numbered) exits all throughout. See these examples which motivated me to make this suggestion: Bay Area `#11/37.5898/-122.2140` <img width="1668" height="952" alt="Image" src="https://github.com/user-attachments/assets/2cce2708-ed99-4881-bbf8-23d2dae4a654" /> Bay Area `#12/37.7004/-122.3712` <img width="1668" height="952" alt="Image" src="https://github.com/user-attachments/assets/217e351f-dd30-49d5-904c-1afed82e3d8c" /> Los Angeles `#11/34.0054/-118.2060` <img width="1668" height="952" alt="Image" src="https://github.com/user-attachments/assets/db19d882-2abc-4445-9c00-fa04fec8b715" /> San Diego `#11/32.8150/-117.0271` <img width="1668" height="952" alt="Image" src="https://github.com/user-attachments/assets/eb4de413-d22c-4acd-9e7c-cb28677c5d25" /> I realize this is a judgement call so I have no data or math to back this up. But in my opinion the exit numbers are too cluttered, at too high of a zoom level. Related to #4748. I also asked this on [Reddit](https://old.reddit.com/r/openstreetmap/comments/1sc2kno/). As we draw less exit numbers, we can see more highway number badges - my best example of this is the Bay Area Z12 image above, where I think the map is clearly more legible with the highway shields drawn. I would suggest to change this: https://github.com/openstreetmap-carto/openstreetmap-carto/blob/0235e0f8d21630ff564e8fd4bbf9e700d14adab8/style/roads.mss#L3386-L3387 from `zoom >= 11` to `zoom >= 13`. Thoughts? Thank you for reading