docs: update GPG keys

[Jcahilltorre]

Proposed changes

Update the NGINX GPG keys in the agent docs

Before:

When prompted to accept the GPG key, verify that the fingerprint matches 573B FD6B 3D8F BC64 1079 A6AB ABF5 BD82 7BD9 BF62, and if so, accept it.

After:

When prompted to accept the GPG key, verify that the fingerprint matches 8540 A6F1 8833 A80E 9C16 53A4 2FD2 1310 B49F 6B46, 573B FD6B 3D8F BC64 1079 A6AB ABF5 BD82 7BD9 BF62, 9E9B E90E ACBC DE69 FE9B 204C BCDC D8A3 8D88 A2B3, and if so, accept it.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• 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
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[github-actions]

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