doc: use "is" rather than "has been"

In deprecations.md, there are many uses of "has been" that are mildly awkward. This changes several instances of things like "fhqwhgads() has been deprecated" to "fhqwhgads() is deprecated".

Especially for non-native speakers, present tense is generally easier to read and understand than past participles. (Or at least that's my experience reading things in languages other than English.)

Checklist

• documentation is changed or added
• commit message follows commit guidelines