Remove/replace "performance review" language

Why is this change being made?

"Performance review" is a psychologically loaded term - many people dread having quarterly/annual performance reviews. And GitLab does not (any longer) practice performance reviews (although it does practice 360 Feedback), so references to performance reviews in the handbook are unnecessary.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what