ci: Do not suppress output of "make mockery" (#239)

Looking at Dependabot updates like #233, it makes no sense that the code generation check is failing, because running `make mockery` locally on that dependabot branch works just fine.

This should hopefully help debug what's going on there.

---

#### PR checklist

- [x] Tests written/updated, or no tests needed
- [x] `CHANGELOG_PENDING.md` updated, or no changelog entry needed
- [x] Updated relevant documentation (`docs/`) and code comments, or no
      documentation updates needed

.github/workflows/check-generated.yml

@@ -32,7 +32,7 @@ jobs:
         run: |
           set -euo pipefail
 
-          make mockery 2>/dev/null
+          make mockery
 
           if ! git diff --stat --exit-code ; then
             echo ">> ERROR:"