net-im/signal-cli: Update to 0.6.10

Release notes:

• https://github.com/AsamK/signal-cli/releases/tag/v0.6.9
• https://github.com/AsamK/signal-cli/releases/tag/v0.6.10

Port changes:

• Improve the comment about jffi-native to be a bit more clear.
• Set JAVA_HOME for Gradle explicitly. Gradle was not picking up the right Java version automatically.
• Remove post-patch; it is no longer needed.
• Bump the required Java version to 11+ as documented by the upstream.