pgp key retrieval requires auth

[michel-laterman]

The PGP retrieval endpoint requires auth from the client:

fleet-server/internal/pkg/api/handlePGPRequest.go

Lines 54 to 58 in 76abe76

	key, err := authAPIKey(r, pt.bulker, pt.cache)
	if err != nil {
	return err
	}
	zlog = zlog.With().Str(LogEnrollAPIKeyID, key.ID).Logger()

While this is defined behaviour as part of the OpenAPI spec

fleet-server/model/openapi.yml

Lines 1762 to 1768 in 76abe76

	/api/agents/upgrades/{major}.{minor}.{patch}/pgp-public-key:
	get:
	operationId: getPGPKey
	summary: retrieve a PGP key from the fleet-server's local storage.
	description: "Get a PGP key that can be used to verify agent upgrades. Key is stored on (fleet-server's) disk."
	security:
	- apiKey: []

it should be removed as agents do not include an API key when attempting to get the PGP key