warn on `errorMessageMaxLength` deprecated config option usage

[trentm]

The errorMessageMaxLength option was deprecated by longFieldMaxLength in #2193. The option should be removed for the next major version.

Update (2023-07): Let's just warn with process.emitWarning (... or log.warn?)

[david-luna]

As discussed we will log.warn

[trentm]

This was done in #3553 on the "dev/4.x" branch, which will be the active 4.x release branch soonish.