RFR: 7903790: Use source instead of release option for library compilation

[Christian Stein]

Please review this change to use `--source=N` instead of `--release=N` in order to prevent compiler errors reading `exporting a package from system module java.base is not allowed with --release` when compiling a library using preview features, enabled via the new `LIBRARY.properties` configuration file.

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Commit messages:
 - 7903790: Use source instead of release option for library compilation

Changes: https://git.openjdk.org/jtreg/pull/221/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=221&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903790
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/221.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/221/head:pull/221

PR: https://git.openjdk.org/jtreg/pull/221