[jdk11u-dev] RFR: 8207248: Reduce incidence of compiler.warn.source.no.bootclasspath in javac tests

[Goetz Lindenmaier]

I backport this for parity with 11.0.19-oracle.

Change itself applied clean, but
WeirdTwr.java needs fix from 8206439: Remove javac -source/-target 6 from langtools regression tests
Else it fails after delting the .out file.

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Commit messages:
 - Backport a376d5dab4961ed5d9097ddf7010c64e14fdc9ec

Changes: https://git.openjdk.org/jdk11u-dev/pull/1721/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1721&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8207248
  Stats: 42 lines in 18 files changed: 0 ins; 31 del; 11 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1721.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1721/head:pull/1721

PR: https://git.openjdk.org/jdk11u-dev/pull/1721