RFR: 8302979: Simplify FileChannelLinesSpliterator.SUPPORTED_CHARSETS and its usage

[Sergey Tsypanov]

Code of `Files.lines()` could be slightly simplified:

- use charsets instead of their names
- use `Set.of()` instead of `HashSet` in `FileChannelLinesSpliterator`

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Commit messages:
 - Simplify FileChannelLinesSpliterator.SUPPORTED_CHARSETS

Changes: https://git.openjdk.org/jdk/pull/12688/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12688&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302979
  Stats: 11 lines in 2 files changed: 0 ins; 3 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/12688.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12688/head:pull/12688

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