[kokoro] Add EL10 signing key to release config

[braydonk]

Description

Starting with RHEL 10, we are expected to sign with a specific key for each major RHEL version. This PR adds the new key to our release configs and adjusts our signing script to set up the proper signing key based on the current distro.

Related issue

b/444772452

How has this been tested?

All I can do is send it to CI and hope it works. :)

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.