RFR: 8302603: Use Set.of in java.nio.charset.Charset

[Glavo]

A small patch, using `Set.of` instead of `Collections.unmodifiableSet()` of `Charset#aliasSet`.

The alias should not be null, and the aliasSet is an immutable copy, so it is safe to use `Set.of`.

I have investigated all charsets, most of charsets have no more than 4 aliases, and the maximum is only 17.

In this case, the performance difference between immutable sets and HashSet can be ignored, and the memory footprint of immutable sets is smaller.

I need someone to help me open an Issue on JBS. Thank you very much.

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Commit messages:
 - Use Set.of in Charset

Changes: https://git.openjdk.org/jdk/pull/12570/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12570&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302603
  Stats: 8 lines in 1 file changed: 0 ins; 6 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12570.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12570/head:pull/12570

PR: https://git.openjdk.org/jdk/pull/12570