[28] JEP 535: Shenandoah GC: Generational Mode by Default

ref: https://openjdk.org/jeps/535
Summary
Switch the default mode of the Shenandoah Garbage Collector to the generational mode. Deprecate the non-generational mode, with the intent to remove it in a future release.

ref #4051 for an earlier work - this is was in 25.

Investigate for any impact on JDT.Core -should not affect ideally - added for completeness's sakle

Start by reading JEP 535 and the earlier work in issue #4051, then inspect JDT.Core for assumptions about Shenandoah or generational GC modes. Done means identifying any JDT.Core impact and documenting or addressing the required change, if one exists.

Written by the indexing model from the issue text.