games/chessx: Update to 1.6.0.

• Volunteer to be a maintainer
• Do not include obsolete i18n: see https://sourceforge.net/p/chessx/wiki/HOWTO%20Translate/
• Point to the proper license file
• Fix bug preventing compilation if USE_SPEECH is not defined

Changelog: https://github.com/Isarhamster/chessx/blob/master/ChangeLog.md#may-2023-version-160

PR: 271348