chore: sign off release-please commits and pin action SHA (#99)

Adds the `signoff` input to release-please-action so the bot's release
PRs pass the DCO check, and pins the action to a SHA for v5.

Signed-off-by: Todd Baert <todd.baert@dynatrace.com>

Author: toddbaert

.github/workflows/release-please.yml

@@ -17,9 +17,10 @@ jobs:
 
     # Release-please creates a PR that tracks all changes
     steps:
-      - uses: googleapis/release-please-action@v5
+      - uses: googleapis/release-please-action@45996ed1f6d02564a971a2fa1b5860e934307cf7 # v5
         with:
           token: ${{ secrets.RELEASE_PLEASE_ACTION_TOKEN }}
+          signoff: 'OpenFeature Bot <109696520+openfeaturebot@users.noreply.github.com>'
 
   # TODO: publishing goes here
   # publish: