Problems upgrading gems

As it stands several key gems cannot be upgraded because of version conflicts. Specifically, v4 of `sinatra` depends on `rack (>= 3, < 4)` and that conflicts with `sprockets-sass`. This is because DevDocs depends on `sprockets-sass`, but the latest version of _that_ is only compatible with `sprockets < 4` and finally `sprockets 3.75` (the highest version compatible with `sprockets-sass`) requires `rack <3`.

In short, we can't upgrade `sinatra` or `rack` because there's no compatible version of `sprockets-sass` and that's unmaintained at this point.

## What sprocket-sass is doing

It enables `@import` statements to work in .scss files, including when the imported file is an ERB that in turn compiles to SCSS (that compiles to CSS). The main place this happens is
https://github.com/freeCodeCamp/devdocs/blob/2c5b5bd25985101fa2f06c28bb9bae15d821bfbc/assets/stylesheets/application.css.scss

## What can be done

I see two potential paths forward

1. Fork `sprockets-sass` and make it compatible with `sprockets` 4 (and Ruby 3)
2. Inline the imports and convert `application.css.scss` to `application.css.scss.erb`, which _should_ work since the issue `sprockets-sass` solved was specifically `@importing` files, not compiling .erb files.

I'm not sure how long 1. would take. I looked into it, but I didn't make much headway.

Option 2. would be ugly, but not awful. Reason being, the imported files are not used more than once, so the code wouldn't get any less DRY. We would end up with one massive file, though.

从 issue 中描述的依赖约束开始，检查 assets/stylesheets/application.css.scss，尤其是其中对 @import 的使用。确定项目应该替换 sprockets-sass，还是修改该 stylesheet，然后验证所选择的方案允许升级 Sinatra、Rack、Sprockets 和 Ruby。完成的标准是解决依赖冲突，同时不破坏 stylesheet 编译。

由索引模型根据 Issue 内容生成。