midx: rename `get_midx_checksum()` to `midx_get_checksum_hash()`

midx: rename `get_midx_checksum()` to `midx_get_checksum_hash()`

Since 541204aabea (Documentation: document naming schema for structs and
their functions, 2024-07-30), we have adopted a naming convention for
functions that would prefer a name like, say, `midx_get_checksum()` over
`get_midx_checksum()`.

Adopt this convention throughout the midx.h API. Since this function
returns a raw (that is, non-hex encoded) hash, let's suffix the function
with "_hash()" to make this clear. As a side effect, this prepares us
for the subsequent change which will introduce a "_hex()" variant that
encodes the checksum itself.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>