RFR: 8302979: (fs) Files usage of SUPPORTED_CHARSETS could be simplified [v2]

[Sergey Tsypanov]

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

Sergey Tsypanov has updated the pull request incrementally with one additional commit since the last revision:

  Fix indentation

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12688/files
  - new: https://git.openjdk.org/jdk/pull/12688/files/a4a2b389..51803254

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12688&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12688&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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