"March 31, 2021 ... is the date set under ... the Consolidated Appropriations Act, 2021 as the snapshot date on which a partial plan termination may be avoided through rehires that restore earlier plan participation levels. Specifically, a plan will not be treated as having experienced a partial plan termination if on March 31, 2021, the number of active plan participants is at least 80 percent of the number covered by the plan on March 13, 2020[.]"