Upgrade mongodb driver to 1.5.1 to resolve security issue in external dependency.

[RichVanderwal]

This should be a quick fix. However, there's one puzzling thing: this test passes in CI, but on my workstation fails with both 1.5.1 and 1.0.0, which is what the integration's go.mod file is set to use.

We should probably figure this out before committing this change.

√ ~/newrelic/go-agent/v3/integrations/nrmongo % go test ./...                                                                                                               14:21:54
# github.com/newrelic/go-agent/v3/integrations/nrmongo [github.com/newrelic/go-agent/v3/integrations/nrmongo.test]
./nrmongo_test.go:44:32: not enough arguments in call to sysinfo.Hostname
	have ()
	want (bool, []string)
./nrmongo_test.go:242:7: reply.SetSampleEverything undefined (type *internal.ConnectReply has no field or method SetSampleEverything)
FAIL	github.com/newrelic/go-agent/v3/integrations/nrmongo [build failed]
?   	github.com/newrelic/go-agent/v3/integrations/nrmongo/example	[no test files]
FAIL