Rename math.log -> math.logb

[expikr]

Out of @Snektron's suggestion in #19218 -- if math.ln is a problematic name to use for the complex-generic natural logarithm, then the existing arbitrary-base logarithm should be renamed to math.logb so that natural logarithm can use the name math.log; otherwise, if math.ln is fine then this PR and the corresponding issue should be closed.

Closes #19252