Add v5 endpoint for drain-controller to nginx.conf

[fhambrec]

This change opens a new endpoint in the nginx.conf that was added to the internal syslog_drain_controller lately.
The PR that introduces the new endpoint can be found here.

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[philippthun]

This new endpoint should be forbidden for external clients, as it's done for /internal/v4/.

capi-release/jobs/cloud_controller_ng/templates/nginx_external_endpoints.conf.erb

Lines 1 to 4 in 4bf9df6

	# Forbid access to internal endpoints
	location /internal/v4/ {
	return 403 'Forbidden';
	}