Warmup and EngineVersion don't close cache on failed lookup

[enocom]

EngineVersion doesn't close the cache on error:

cloud-sql-go-connector/dialer.go

Lines 387 to 396 in c4fb69f

	cn, err := instance.ParseConnName(icn)
	if err != nil {
	return "", err
	}
	i := d.connectionInfoCache(cn, nil)
	ci, err := i.ConnectionInfo(ctx)
	if err != nil {
	return "", err
	}
	return ci.DBVersion, nil

Warmup doesn't close the cache on error:

cloud-sql-go-connector/dialer.go

Lines 403 to 412 in c4fb69f

	cn, err := instance.ParseConnName(icn)
	if err != nil {
	return err
	}
	cfg := d.defaultDialConfig
	for _, opt := range opts {
	opt(&cfg)
	}
	_ = d.connectionInfoCache(cn, &cfg.useIAMAuthN)
	return nil

As a result, if a client connects with a bad instance connection name, missing IAM permissions, no network connectivity etc, the background refresh will continue running regardless.

Instead, we should close the cache and delete it from the shared map.