RFR: CODETOOLS-7903088: Update version to 7; update build JDK to 11

[Jonathan Gibbons]

Please review a change to increase the `jtreg` version to 7 (simple) and the JDK required to build `jtreg` to JDK 11 or newer. (not so simple).

A consequence of using a more recent version of JDK to build `jtreg` is that we can (going forward) start to modernize the code, but it does mean that starting in this version (7), we can no longer run tests on JDK older than JDK 8.   To run tests on older versions of JDK, you will have to use an older version of `jtreg`.

There was one gotcha in moving to a newer JDK.  Some older versions/updates of JDK do not set the correct value of the system property `os.version` on a Mac.  This is fixed in JDK 17, and the fix has been back ported to most versions from 11 to 16.   For now, there are various checks in the `build.sh` script, the `Makefile`, and in `jtreg` itself, to make sure it does not detect the bug in the JDK used to build or run `jtreg`.   This code can be removed when we move forward (again, later) to using JDK 17 to build `jtreg`, but for now, "baby steps".

A number of tests needed to be updated, typically to specify a newer version of JDK to run `jtreg`, while leaving unchanged the version of JDK used in the tests themselves.

-------------

Commit messages:
 - CODETOOLS-7903088: Update version to 7; update build JDK to 11

Changes: https://git.openjdk.java.net/jtreg/pull/48/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=48&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903088
  Stats: 224 lines in 27 files changed: 117 ins; 17 del; 90 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/48.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/48/head:pull/48

PR: https://git.openjdk.java.net/jtreg/pull/48