Revisit unicode-to-unicode confusables in RUF001, RUF002, RUF003

[dylwil3]

In #8473 the RUF rules regarding confusable/ambiguous unicode were extended, in preview, to include cases where one might confuse some non-ASCII unicode character with another (as opposed to only treating cases where a non-ASCII unicode character may be confused for an ASCII character).

However, the determination of what counts as "confusable", or which of a confusable pair may be preferred, needs some re-examination. The main example encountered in an ecosystem check is the Greek letter μ, which may be used as a variable name (in which case one wants the Greek letter U+03BC) or as part of an SI unit like µs (in which case one wants the "micro" symbol U+00B5).

See the comments on #14403 .