RFR: 8260245: Update ICU4C to version 68.2

[Arun Joseph]

We currently use ICU4C version 64.2. We should update to the latest stable version 68.2.
http://site.icu-project.org/home

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

Commit messages:
 - Add UPDATING.txt file
 - Update license
 - Modify exec files
 - Add icudt68l repo
 - Remove icudt file
 - 8260245: Update ICU4C to version 68.2

Changes: https://git.openjdk.java.net/jfx/pull/456/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=456&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260245
  Stats: 45770 lines in 673 files changed: 24662 ins; 8082 del; 13026 mod
  Patch: https://git.openjdk.java.net/jfx/pull/456.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/456/head:pull/456

PR: https://git.openjdk.java.net/jfx/pull/456