GPG key issues causing package update failures

Start with the failed package update output for planet.nyc1.psf.io and review the referenced Python issue #568 for the Ubuntu 18.04 and Python 2 constraints. Trace where the datadog, nginx, and Salt repository keys are configured; done means package updates no longer fail signature verification without breaking the host's supported setup.

May be able to fix by updating to new keys (datadog, nginx, salt) but also this host is far out of support range (18.04) but is stuck due to python2 (See: https://github.com/python/planet/issues/568#issue-2417204196)

Summary for planet.nyc1.psf.io
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Succeeded: 161 (changed=3)
Failed:      3

...
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          ID: python-requests
    Function: pkg.latest
      Result: False
     Comment: An error was encountered while checking the newest available version of package(s): W: GPG error: http://nginx.org/packages/mainline/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ABF5BD827BD9BF62
              E: The repository 'http://nginx.org/packages/mainline/ubuntu bionic InRelease' is not signed.
              W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://apt.datadoghq.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E6266D4AC0962C7D
              W: GPG error: https://packages.broadcom.com/artifactory/saltproject-deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 64CBBC8173D76B3F
              E: The repository 'https://packages.broadcom.com/artifactory/saltproject-deb stable InRelease' is not signed.
     Started: 18:17:06.302980
    Duration: 6061.297 ms
     Changes:
...
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          ID: consul-external-console
    Function: consul.external_service
        Name: console
      Result: False
     Comment: One or more requisite failed: base.salt.python-requests
     Started: 18:17:13.456682
    Duration: 0.012 ms
     Changes:
----------
          ID: consul-external-jobs
    Function: consul.external_service
        Name: jobs
      Result: False
     Comment: One or more requisite failed: base.salt.python-requests
     Started: 18:17:13.457442
    Duration: 0.005 ms
     Changes:
----------
...