[GRPC] Bump org.opensearch:protobufs to 0.18.0 and fix compilation errors

[karenyrx]

Description

1. Bump up the protobuf version
2. Fix compilation errors.
3. Fix a bug for Inner Hits.
4. Remove Sort-related protobufs as they are incorrect and will be fixed in a future PR.

Related Issues

Part of #19446

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

✅ Gradle check result for 28b984c: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 95.16129% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.98%. Comparing base (28689b7) to head (fef4f49).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...oto/request/search/InnerHitsBuilderProtoUtils.java	94.44%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19447      +/-   ##
============================================
+ Coverage     72.96%   72.98%   +0.02%     
- Complexity    69969    69996      +27     
============================================
  Files          5676     5676              
  Lines        321265   321246      -19     
  Branches      46445    46439       -6     
============================================
+ Hits         234407   234460      +53     
+ Misses        67901    67830      -71     
+ Partials      18957    18956       -1     

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[github-actions]

✅ Gradle check result for fef4f49: SUCCESS