ci(lint): enforce commit signature on PR commits

The signature check was disabled while it required a local keyring. Since
v0.20.0 it resolves the author through the GitHub API and verifies against
the user's published GPG/SSH keys, so the action now works in plain CI
without setup. Drop the disable so PRs are required to be signed.

Signed-off-by: Nerijus Bendžiūnas <nerijus.bendziunas@gmail.com>

Author: benner

.github/workflows/lint-commits.yml

@@ -25,4 +25,3 @@ jobs:
         uses: benner/commit-guard@cab0d35c41a3f99fc5f61895f491d0ce3f5aff1c  # v0.20.0
         with:
           range: origin/${{ github.base_ref }}..HEAD
-          disable: signature