Change the term "terminations" to "offboarding"

Why is this change being made?

During the CEO Shadow program, Sid had a great suggestion to change the word `termination` to `offboarding`. This will change on all other documentation and sheets being used for offboarding, as well.

cc @dstanley @emilie

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