openhtmltopdf-rtl-support: Upgrade icu4j to > 63.1 to mitigate CVE-2018-18928

openhtmltopdf-rtl-support currently uses icu4j v59.1 which seems to be affected by the vulnerability.
The mentioned security vulnerability https://nvd.nist.gov/vuln/detail/CVE-2018-18928 states, that this applies to the C-Version, but the fix also contained changes to Java files - so I assume it would be better to update the Java library used.

openhtmltopdf-rtl-support compiles against the latest version, 71.1 and my tests with some RTL text did not find any issues so I guess the version could just be bumped.