knex instrumentation should not *statically* disable based on `spanStackTraceMinDuration` config value

Knex instrumentation is about improving possible span stack traces. It currently will not enable if the config says to not gather span stack traces:
https://github.com/elastic/apm-agent-nodejs/blob/5c372b6c60d18ed797ad3f68c348dd93cdd6ebdc/lib/instrumentation/modules/knex.js#L22-L26

However, `spanStackTraceMinDuration` can be updated via central-config, so this disabling should only be done *dynamically*