Use pattern matching for instanceof where appropriate

[wilkinsona]

if (obj instanceof String) {
    String s = (String) obj;
    // use s
}

becomes

if (obj instanceof String s) {
    // use s
}

[avillalain]

@wilkinsona Is this issue something that new comers can start working on?

[wilkinsona]

Not at the moment. Thanks for the offer, though. We haven't started work on 3.0 yet and there are a number of things that'll have to be tackled before we can look at this one.

[wilkinsona]

Closing in favor of #31475.