Update rubygems public key setting expiry to 20260707

[gaiksaya]

Description

The current public key is about to expire in few days (Jul 26 23:22:27 2025 GMT)
This PR updates the public key to newly generated one.

openssl x509 -text -noout -enddate -in certs/opensearch-rubygems.pem
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 1 (0x1)
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: CN=opensearch, DC=amazon, DC=com
        Validity
            Not Before: Jul  7 20:02:25 2025 GMT
            Not After : Jul  7 20:02:25 2026 GMT

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has documentation added
• Commits are signed as per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.