JDK HTTP Client: Rework expect-continue workaround

[kwin]

Affected version

2.0.14

Bug description

In #1112 there was a workaround implemented to not use expect-continue prior Java 20 due to https://bugs.openjdk.org/browse/JDK-8286171. Meanwhile the fix has been backported to 17.0.17 (https://bugs.openjdk.org/browse/JDK-8364017), so would be good to allow setting expect-continue also on those JDKs.