fix(deps): upgrade msgpack to 1.2.2 for security fix (dev only)

Resolves a high severity Dependabot alert: out-of-bounds read/crash on
Unpacker reuse after a caught error. Transitive dev dependency via
cachecontrol (pip-audit).

GHSA-6v7p-g79w-8964

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: matrixise

requirements/dev.txt

@@ -126,7 +126,7 @@ modelsearch==1.1.1
     # via
     #   -c requirements/main.txt
     #   wagtail
-msgpack==1.1.2
+msgpack==1.2.2
     # via cachecontrol
 openpyxl==3.1.5
     # via