Simplify lease extension logic by using `extract_if()`

[dbolduc]

google-cloud-rust/src/pubsub/src/subscriber/lease_state/exactly_once.rs

Line 92 in 6266932

pub fn retain(&mut self, max_lease_extension: Duration) -> Vec<Vec<String>> {

This logic could be simplified if we used extract_if(). The only problem is it was added in Rust version 1.88, which is higher than our MSRV.

When we bump to MSRV of 1.88, we should revisit this code.

#4880 (comment)