[jdk21u] RFR: 8311943: Cleanup usages of toLowerCase() and toUpperCase() in java.base

[Glavo]

8311943: Cleanup usages of toLowerCase() and toUpperCase() in java.base

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Commit messages:
 - Backport b32d6411c406608ba5f7d60bfb8d935adb876564

Changes: https://git.openjdk.org/jdk21u/pull/205/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=205&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311943
  Stats: 44 lines in 11 files changed: 6 ins; 0 del; 38 mod
  Patch: https://git.openjdk.org/jdk21u/pull/205.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/205/head:pull/205

PR: https://git.openjdk.org/jdk21u/pull/205