change default values for v0.30 release

[Naphthalin]

Changed default values for v0.30 release:

• mlh-threshold to 0.8 as we now have a continuous behaviour. Training runs explicitly set threshold to 0, so no issues there.
• centipawn fallback for WDL_mu to only kick in if W or L < 0.2%, which should happen virtually never. Will still behave jumpy if around that threshold and needs a proper fix at some point, but eval at this point doesn't matter anyway anymore,
• default scoretype to WDL_mu. Thanks to #1898, WDL_mu now is always calculated properly (even if contempt is disabled), and we use it everywhere anyway.

[yuzisee]

Out of curiosity, are https://lczero.org/play/flags/ and https://lczero.org/dev/wiki/lc0-options/ treated differently in terms of when each can (should) be updated after a new release? Neither mentions WDL_mu (yet) but as far as I can tell: one of them requires a pull request, whereas anyone (even me) can edit the other.

[mooskagh]

Out of curiosity, are lczero.org/play/flags and lczero.org/dev/wiki/lc0-options treated differently in terms of when each can (should) be updated after a new release? Neither mentions WDL_mu (yet) but as far as I can tell: one of them requires a pull request, whereas anyone (even me) can edit the other.

The /wiki/ pages are imported from https://github.com/LeelaChessZero/lc0/wiki, while other pages (btw this is the correct link to create the PR) require git repository change. Both pages are intended to be easily changeable by everyone, but we had an incident of a spammer secretly adding links to malware to the page, so now changes have to go through the second pair of eyes (and it's generally useful, e.g. to spot spelling mistakes).