Set $wgCategoryCollation to 'xx-uca-et' on all Estonian-language wikis

We're using a custom tweaked collation instead of 'uca-et' because
'V' and 'W' sort incorrectly (see bug for details).

Bug: 54168
Change-Id: I61a6fd4a472aa0373e02e79a780c6069efbcb230