Use the recommended place for saving APT keys

[hongquan]

Proposed changes

According to apt-key manpage, if the APT keys are downloaded and installed manually, they should be placed in /etc/apt/keyrings/. This PR makes changes to follow it.

Checklist

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• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).