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

[Glavo]

Clean up misuses of `toLowerCase()`/`toUpperCase()` in java.base.

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

Commit messages:
 - Update DateTimeFormatterBuilder
 - Avoid locale-sensitive case conversions in java.base

Changes: https://git.openjdk.org/jdk/pull/14763/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14763&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311943
  Stats: 51 lines in 13 files changed: 9 ins; 0 del; 42 mod
  Patch: https://git.openjdk.org/jdk/pull/14763.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14763/head:pull/14763

PR: https://git.openjdk.org/jdk/pull/14763