Remove fields in cabal spec 3.0

- extensions
- (singular) hs-source-dir
- build-tools

Example, with `cabal-version: 2.5`

```
[laptop] tree-diff % cabal new-build --dry
Errors encountered when parsing cabal file ./tree-diff.cabal:

tree-diff.cabal:59:3: error:
The field "extensions" is removed in the Cabal specification version 2.5. Please use 'default-extensions' or 'other-extensions' fields.

   58 | library
   59 |   extensions: CPP
      |   ^

tree-diff.cabal:60:3: error:
The field "extensions" is removed in the Cabal specification version 2.5. Please use 'default-extensions' or 'other-extensions' fields.

   59 |   extensions: CPP
   60 |   extensions: DeriveGeneric
      |   ^
```

I needed to add new CabalSpecVersion to properly issue deprecation
messages of `extensions`.