Review all Encoding usage for BOM compatibility

[paulirwin]

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Task description

Java's StandardCharsets.UTF_8 does not write a Byte-Order Mark (BOM), while .NET's System.Text.Encoding.UTF8 does include a BOM by default. We have ensured that the IOUtils.CHARSET_UTF_8 does not include a BOM to match Java, and as part of #1018 we've added an internal Support class to allow for using StandardCharsets.UTF_8, but we need to review all usage of System.Text.Encoding.UTF8 to determine if it should be replaced with StandardCharsets.UTF_8 or IOUtils.CHARSET_UTF_8 (whatever best matches the corresponding Java Lucene code) to avoid BOM issues.