<i18n dev> RFR: 8304982: Emit warning for removal of `COMPAT` provider

Mon Apr 3 16:56:12 UTC 2023

This is a precursor to the future removal of the `COMPAT` locale data provider. Before the actual removal of the provider, warn the users who explicitly specify `COMPAT` at the command line in order for their smooth migration to CLDR. A CSR has also been drafted.

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Commit messages:
 - clean-up
 - Added @implNote. Check "JRE" as well. Clean-up
 - Spec change to LocaleServiceProvider
 - clean-up
 - added a test
 - 8304982: Emit warning for removal of `COMPAT` provider

Changes: https://git.openjdk.org/jdk/pull/13302/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13302&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304982
  Stats: 91 lines in 4 files changed: 83 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13302.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13302/head:pull/13302

PR: https://git.openjdk.org/jdk/pull/13302