RFR: 6899: Upgrade core to 8

[Marcus Hirt]

For some reason the Eclipse compiler fails some type inferences when switching to JDK 8, so had to add some
(unnecessary) casts. (Also set JDK 11 as required for running the application builds.)

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Commit messages:
 - 6899: Upgrade core to 8

Changes: https://git.openjdk.java.net/jmc/pull/120/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=120&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6899
  Stats: 155 lines in 43 files changed: 26 ins; 4 del; 125 mod
  Patch: https://git.openjdk.java.net/jmc/pull/120.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/120/head:pull/120

PR: https://git.openjdk.java.net/jmc/pull/120