Bump gitleaks version and update lefthook.yml
After [gitleaks!1504 (Deprecate `detect` and `protect`. Add `git`, `dir`, `stdin`)](https://github.com/gitleaks/gitleaks/pull/1504) & [gitleaks!1533 (Fix behaviour with `--pre-commit` and `--staged`)](https://github.com/gitleaks/gitleaks/pull/1533) we should (eventually) update _both_ our [lefthooks](https://gitlab.com/search?group_id=9970&nav_source=navbar&project_id=278964&repository_ref=master&scope=blobs&search=%22gitleaks%22+path%3Alefthook.yml&search_code=true) with those new commands.
- [x] @katrinleinweber-gtlb: Test with [v8.20](https://github.com/gitleaks/gitleaks/releases/tag/v8.20.0) ~~18.4~~
issue