build: migrate to alternative to github.com/knz/strtime

There is a custom implementation of `Strptime`/`Strftime` that is used in the `builtins` package in github.com/knz/strtime. This is a very old library that doesn't get updates and is authored by an ex-CRL employee. There's some questionable C code in this library that has been throwing warnings for years. Most recently, the build broke on some Macs with an XCode update. [This PR](http://github.com/cockroachdb/cockroach/pull/143803) aims to fix that, but this is probably not the last time this will happen. Find an open-source replacement for this, ideally a pure Go one to allow us to cull some CGo.

Jira issue: CRDB-49049