#1783 Upgrade to LanguageTool 6.5

[Hiroshi Miura]

LanguageTool 6.5 is a latest and last version that supports Java Runtime 11.
It has a bug that breaks downstream Gradle/Java project like OmegaT. We can fork and fix it and update OmegaT to use it.

We have already forked and maintained LT under omegat umbrella.
The PR is at github.com/omegat-org/omegat/pull/1492