RFR: 7903790: Use source instead of release option for library compilation [v2]

[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.

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

  Add explanation

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Changes:
  - all: https://git.openjdk.org/jtreg/pull/221/files
  - new: https://git.openjdk.org/jtreg/pull/221/files/3756b892..265fcbd1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=221&range=01
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=221&range=00-01

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 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