Tweak Mountain & Extreme Hill Generation

New Mountains have been added in the recent 1.17 beta, and they are truly astounding!  However, the new biomes come with one surprise:  the old Extreme Hills still generate, and much more commonly than the new mountains.  The developers have stated that this is intentional, and it's an understandable decision - if the new, enormous mountains were everywhere, the world would be too cluttered.  However, this risks making extreme hills the most useless and obsolete biome in Minecraft.

Previously, the only two benefits of extreme hills were mining emeralds and building mountaintop structures.  However, both of these can now be done much better in the new mountain biomes.  This means that extreme hills will be generating as commonly as before, but with no purpose, for all of the features in extreme hills are shared with mountains or other biomes.

There are three options for solving this problem:

- Option 1: Make extreme hills a nice place to live without the benefit of astounding heights.  This means covering the hills in grass (no more exposed stone), making the slopes slightly less steep (but just as high) and adding more vegetation (occasional trees, berry bushes).  

- Option 2: Make extreme hills the "foothills" to new mountains.  This means they would generate surrounding the new, tall mountains, so wide existing extreme hills biomes would now have mountains inside of them.

-   Option 3: Make 50% of extreme hills now generate as mountains, a simple but effective solution.