Bump external/coqutil from 421da12 to ac8490b

Bumps [external/coqutil](https://github.com/mit-plv/coqutil) from `421da12` to `ac8490b`.
- [Release notes](https://github.com/mit-plv/coqutil/releases)
- [Commits](mit-plv/coqutil@421da12...ac8490b)

---
updated-dependencies:
- dependency-name: external/coqutil
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>