RFR: 8372951: The property jdk.httpclient.quic.maxBidiStreams should be renamed to jdk.internal

[Daniel Fuchs]

The `jdk.httpclient.quic.maxBidiStreams` is an undocumented system property only used in tests. This property is only useful to control the default HTTP/3 test server implementation. It will never be documented or exposed. It should be clearly labelled as internal.
The HTTP/3 client always provide a value of 0 for the `initial_max_streams_bidi` transport parameter, so the default value that the quic stack uses internally to set the parameter if no value is provided is never used on the client side.

The `H3StreamLimitReachedTest` can be used to verify the change - it won't pass if the property is not correctly configured in the test.

I have also taken this opportunity to add comments to the `H3StreamLimitReachedTest` and the `StreamLimitTest`, to explain the logic of each test.

With this I believe that we can also say that this change will fix [JDK-8365794](https://bugs.openjdk.org/browse/JDK-8365794).

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Commit messages:
 - rename maxBidiStreams property
 - fix CLASS_NAME constant
 - 8365794

Changes: https://git.openjdk.org/jdk/pull/28611/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28611&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372951
  Stats: 62 lines in 4 files changed: 52 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/28611.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28611/head:pull/28611

PR: https://git.openjdk.org/jdk/pull/28611