SERVER-77705 FCV-gated the use of v2 checksums to support downgrading in the middle of an index build. However, this was applied to all uses of sorter spill files, and not just resumable ones. Everything else can unconditionally use v2 checksums as the files will never be read by a different server version.

This has some performance benefits when running with FCV <7.3 and enables some subsequent simplifications to the code. Currently v1 checksum calculation is done in a somewhat complicated way that is not beneficial for resumable index builds, and only using v1 checksums for index builds will make that unnecessary.