[foreign-memaccess+abi] RFR: Avoid using StandardCharset in Java code

[Per Minborg]

According to a comment in `java.nio.charset.StandardCharsets`, the use of this class should be avoided from elsewhere in the `jdk.base` module. This PR proposes removing all such direct use of said class in Java code (but not in JavaDocs).


public final class StandardCharsets {

    // To avoid accidental eager initialization of often unused Charsets
    // from happening while the VM is booting up, which may delay
    // initialization of VM components, we should generally avoid depending
    // on this class from elsewhere in java.base.

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Commit messages:
 - Avoid using StandardCharset in Java code

Changes: https://git.openjdk.org/panama-foreign/pull/864/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=864&range=00
  Stats: 12 lines in 3 files changed: 0 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/864.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/864/head:pull/864

PR: https://git.openjdk.org/panama-foreign/pull/864