RFR: CODETOOLS-7903352: JOL: Drop support for JDK 7

[Aleksey Shipilev]

Head JDK 20-ea does not support building with -source/target 7 anymore. JDK 7 is very old and unlikely to be used widely. We can bump the minimal version to JDK 8 to get... ahem... all the new features that release brings!

If you have a strong opinion that we should *not* drop JDK 7 support, please speak up soon.

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Commit messages:
 - Do not test JDK 7 either
 - Fix

Changes: https://git.openjdk.org/jol/pull/34/files
 Webrev: https://webrevs.openjdk.org/?repo=jol&pr=34&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903352
  Stats: 94 lines in 14 files changed: 1 ins; 40 del; 53 mod
  Patch: https://git.openjdk.org/jol/pull/34.diff
  Fetch: git fetch https://git.openjdk.org/jol pull/34/head:pull/34

PR: https://git.openjdk.org/jol/pull/34