Bump step-security/harden-runner from 2.20.0 to 2.21.0

Bumps [step-security/harden-runner](https://github.com/step-security/harden-runner) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/step-security/harden-runner/releases)
- [Commits](step-security/harden-runner@bf7454d...05e3151)

---
updated-dependencies:
- dependency-name: step-security/harden-runner
  dependency-version: 2.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/push_gem.yml

@@ -27,7 +27,7 @@ jobs:
 
     steps:
       - name: Harden Runner
-        uses: step-security/harden-runner@bf7454d06d71f1098171f2acdf0cd4708d7b5920 # v2.20.0
+        uses: step-security/harden-runner@05e31511f85b41b11d1cf0ef85d0992719546e2c # v2.21.0
         with:
           egress-policy: audit