Upgrade ICU Plugin to latest available in Lucene

[clintongormley]

The ICU plugin currently uses ICU 54.1, while Lucene is on v56.1 and plans to upgrade to v58.1 (https://issues.apache.org/jira/browse/LUCENE-7540)

We should upgrade to the latest available in Lucene for 6.0.

Relates to #21352

[jimczi]

Note that the ICU plugin now uses v56.1:
#24821

This is the version tested by Lucene but not the latest available:
https://issues.apache.org/jira/browse/LUCENE-7540

[jimczi]

Lucene master uses the latest version now (59.1) so we'll have to upgrade the ES plugin too when we upgrade to a new snapshot.