Allow ghc-events 0.21 and optparse-applicative 0.19

Currently fails in Nixpkgs' haskell-updates branch with:

```
Encountered missing or private dependencies:
    ghc-events >=0.20.0 && <0.21,
    optparse-applicative >=0.14.3 && <0.19
```

Builds fine when jailbreaking.

Changelogs:
- https://hackage.haskell.org/package/ghc-events-0.21.0.0/changelog
- https://hackage.haskell.org/package/optparse-applicative-0.19.0.0/changelog