RFR: 8302979: Simplify FileChannelLinesSpliterator.SUPPORTED_CHARSETS and its usage

[Alan Bateman]

On Tue, 21 Feb 2023 11:20:01 GMT, Sergey Tsypanov <stsypanov at openjdk.org> wrote:

> Code of `Files.lines()` could be slightly simplified:
> 
> - use charsets instead of their names
> - use `Set.of()` instead of `HashSet` in `FileChannelLinesSpliterator`

@stsypanov Can you fix the description to match the JBS issue?

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PR: https://git.openjdk.org/jdk/pull/12688