Bump step-security/harden-runner from 2.14.2 to 2.15.0

Bumps [step-security/harden-runner](https://github.com/step-security/harden-runner) from 2.14.2 to 2.15.0.
- [Release notes](https://github.com/step-security/harden-runner/releases)
- [Commits](step-security/harden-runner@5ef0c07...a90bcbc)

---
updated-dependencies:
- dependency-name: step-security/harden-runner
  dependency-version: 2.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/push_gem.yml

@@ -20,7 +20,7 @@ jobs:
     steps:
       # Set up
       - name: Harden Runner
-        uses: step-security/harden-runner@5ef0c079ce82195b2a36a210272d6b661572d83e # v2.14.2
+        uses: step-security/harden-runner@a90bcbc6539c36a85cdfeb73f7e2f433735f215b # v2.15.0
         with:
           egress-policy: audit