Upgrade step-security/harden-runner v2.15.0 -> v2.15.1 (#2147)

This aligns the `assign-milestone` GitHub actions workflow with the
changes in ba8f784.

See:
- https://github.com/step-security/harden-runner/releases/tag/v2.15.1
- step-security/harden-runner@v2.15.0...v2.15.1

Author: Picnic-DevPla-Bot

.github/workflows/assign-milestone.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Install Harden-Runner
-        uses: step-security/harden-runner@a90bcbc6539c36a85cdfeb73f7e2f433735f215b # v2.15.0
+        uses: step-security/harden-runner@58077d3c7e43986b6b15fba718e8ea69e387dfcc # v2.15.1
         with:
           disable-sudo-and-containers: true
           egress-policy: block