Update RediSearch module to v8.9.81

Updates the bundled RediSearch module version used by Redis module
builds from v8.9.80 to v8.9.81.

This picks up the tagged RediSearch 8.9.81 release for the 8.10 MS2
milestone.

Validation:

• Checked that v8.9.81 exists in RediSearch/RediSearch and resolves
  to ccfdde4eb4444049f5965c4aa69a97d9a23a8cf2.
• Checked the diff is limited to modules/redisearch/Makefile.
• Ran git diff --check.

---

Note

Low Risk
Single-version Makefile change with no logic or security surface;
routine upstream module pickup.

Overview
Bumps the RediSearch bundle pin from v8.9.80 to
v8.9.81 via MODULE_VERSION in modules/redisearch/Makefile, so
Redis module builds pull the 8.9.81 tagged upstream release for the
8.10 MS2 milestone.

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