[memprof] Rename "v2" functions and tests (NFC) (#156247)
I'm planning to remove the V2 support. Now, some functions and tests
should not be removed just because they have "v2" in their names.
This patch renames them.
- makeRecordV2: Renamed to makeRecord. This has "V2" in the name
because the concept of call stack ID came out as part of V2. It is
still useful for use with V3 and V4.
- test_memprof_v4_{partial,full}_schema: Upgraded to use V4. These
tests perform serialization/deserialization roundtrip tests of a
MemProfRecord with {partial,full} schema.
