RFR: 8123: Use StandardCharsets where possible

[Christoph Dreis]

Hi,

this PR replaces `UTF-8` String usages with `StandardCharsets.UTF_8` where possible.

I'd appreciate if this is sponsored including a ticket etc.

Cheers,
Christoph

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Commit messages:
 - Use StandardCharsets where possible

Changes: https://git.openjdk.org/jmc/pull/518/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=518&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8123
  Stats: 60 lines in 10 files changed: 33 ins; 14 del; 13 mod
  Patch: https://git.openjdk.org/jmc/pull/518.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/518/head:pull/518

PR: https://git.openjdk.org/jmc/pull/518