RFR: 8368739: [AIX] java/net/httpclient/http3/H3SimpleGet.java#useNioSelector and #with-continuations fail

[Matthias Baesken]

After [JDK-8349910](https://bugs.openjdk.org/browse/JDK-8349910) the 2 tests java/net/httpclient/http3/H3SimpleGet.java#useNioSelector and #with-continuations fail on AIX; all the other HTTP3 related tests seem to do fine on AIX.

The tests fail in the runs with  `-Djdk.httpclient.quic.defaultMTU=16336` .  This setting has to be adjusted to a lower value for AIX to work.

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Commit messages:
 - JDK-8368739

Changes: https://git.openjdk.org/jdk/pull/28041/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28041&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368739
  Stats: 51 lines in 1 file changed: 51 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28041.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28041/head:pull/28041

PR: https://git.openjdk.org/jdk/pull/28041