fix(backup): fix DBRetentionAndShardFromPath parsing error between different OS

[chengshiwen]

Closes #25361

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Use separatorRegexp = regexp.MustCompile(`[/\\]`) to Split instead of strings.Split(path, string(filepath.Separator))

@davidby-influx @gwossum

[chengshiwen]

@bnpfeife @jdstrand @devanbenz

[davidby-influx]

LGTM. Thanks for your contribution.