fix(transport/fc): prevent auto-tuning to decrease max_active
A user can configure the initial receive window size. Given a user that sets the
initial receive window size to a value larger than the maximum auto-tuning
value, the auto-tuning logic should not decrease the max_active value below the
user-configured value.
Note that Firefox currently sets a larger initial receive window size than
the maximum auto-tuning value.
https://github.com/mozilla-firefox/firefox/blob/e1eada69e2ddd86a398ccb141dcbf772254162eb/modules/libpref/init/StaticPrefList.yaml#L15423-L15427
https://github.com/mozilla/neqo/blob/0798e0ac28b31d6a18e3f43801336abeeca195c7/neqo-transport/src/connection/params.rs#L80