chore: Bump memchr to 2.6.0 (#2902)

Fresh off the press, memchr 2.6.0 adds vector search routines for
aarch64. That directly improves our search performance for both text and
regex searches. Per BurntSushi's claims, the simple string searches in
ripgrep got ~2 times faster (more details available in
BurntSushi/memchr#129).

Release Notes:

- N/A