gpg: change warning log statement to debug

Change warning type log statement about exported subkeys to debug.
This is no longer relevant for GPGSigner.import_, which don't
return key bundles to the user, but only a key or a subkey.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

securesystemslib/gpg/common.py

@@ -635,7 +635,7 @@ def get_pubkey_bundle(data, keyid):
     ):
         if public_key and public_key["keyid"].endswith(keyid.lower()):
             if idx > 1:
-                log.warning(
+                log.debug(
                     "Exporting master key '{}' including subkeys '{}' for"  # pylint: disable=logging-format-interpolation,consider-using-f-string
                     " passed keyid '{}'.".format(
                         master_public_key["keyid"],