Update search-index submodule to fixed version

[ZepengLi111]

• Pin search-index to commit: b4acfa57dd26958b3a98cd6ece5991e05e2bbe0a
• Ensure stable dependency for production environment
• Resolve compatibility with current codebase

Changelog category (leave one):

• Build/Testing/Packaging Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Update search-index submodule to fixed version for better stability

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

Information about CI checks: https://clickhouse.com/docs/en/development/continuous-integration/

CI Settings (Only check the boxes if you know what you are doing):

• Allow: All Required Checks
• Allow: Stateless tests
• Allow: Stateful tests
• Allow: Integration Tests
• Allow: Performance tests
• Allow: All Builds
• Allow: batch 1, 2 for multi-batch jobs
• Allow: batch 3, 4, 5, 6 for multi-batch jobs

---

• Exclude: Style check
• Exclude: Fast test
• Exclude: All with ASAN
• Exclude: All with TSAN, MSAN, UBSAN, Coverage
• Exclude: All with aarch64, release, debug

---

• Run only fuzzers related jobs (libFuzzer fuzzers, AST fuzzers, etc.)
• Exclude: AST fuzzers

---

• Do not test
• Woolen Wolfdog
• Upload binaries for special builds
• Disable merge-commit
• Disable CI cache