The `sstable validate-checksums` tool provides the validation result
via the `valid_checksums` key in its JSON response.
The name can be misleading as it refers to both the per-chunk checksums
and the digest (full checksum). We use the terms "digest" and
"full checksum" interchangeably.
Replace with the word "valid" to avoid confusion.
Signed-off-by: Nikos Dragazis <nikolaos.dragazis@scylladb.com>