use newer ghc in ci

[parsonsmatt]

Before submitting your PR, check that you've:

• [ ] Bumped the version number.
• [ ] Documented new APIs with Haddock markup.
• [ ] Added @since declarations to the Haddock.
• [ ] Ran stylish-haskell and otherwise adhered to the style guide.

After submitting your PR:

• [ ] Update the Changelog.md file with a link to your PR.
• [ ] Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts).