RFR: 7903809: Invalid source release N with --enable-preview [v4]

[Christian Stein]

> Please review this change to prevent "invalid source release N with --enable-preview" errors for when an explicit compile task targets a different JDK feature release number then the JDK under test (running the compilation).

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Simplify source/release version detection

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Changes:
  - all: https://git.openjdk.org/jtreg/pull/226/files
  - new: https://git.openjdk.org/jtreg/pull/226/files/c9ab9fbf..2350623c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=226&range=03
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=226&range=02-03

  Stats: 13 lines in 2 files changed: 3 ins; 4 del; 6 mod
  Patch: https://git.openjdk.org/jtreg/pull/226.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/226/head:pull/226

PR: https://git.openjdk.org/jtreg/pull/226