Update jdt.core to I20210503-1800

* re-fix https://bugs.eclipse.org/bugs/show_bug.cgi?id=561820#c12 (JDT)
* more re: improve/differentiate keyword completion
  - revert previous test adjustments
* update tests to JavaSE-11, to match the BREE of o.e.equinox.common
21 files changed