Update RediSearch module to v8.9.80

Jira: https://redislabs.atlassian.net/browse/MOD-16291

Updates the bundled RediSearch module version used by Redis module
builds from v8.8.0 to v8.9.80.

This picks up the tagged RediSearch 8.9.80 release.

Validation:

• Checked that v8.9.80 exists in RediSearch/RediSearch and resolves
  to 280c3ce1bd65fd7306cae75a1d54bce3a05518e4.
• Checked the diff is limited to modules/redisearch/Makefile.
• Ran git diff --check.

---

Note

Low Risk
Single Makefile version pin with no application logic changes; main
risk is behavioral differences in the upstream RediSearch 8.9.80 release
at runtime.

Overview
RediSearch module version bump from v8.8.0 to v8.9.80 via
MODULE_VERSION in modules/redisearch/Makefile, so Redis module
builds fetch and compile the tagged 8.9.80 release instead of 8.8.0.

No other build flags or paths change in this PR.

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