refactor: replace boost predicates with absl

[isotauon]

Summary

• Replace boost::starts_with with absl::StartsWith in src/nameserver/name_server_impl.cc (3 sites).
• Replace boost::iequals with absl::EqualsIgnoreCase in src/tablet/tablet_impl.cc (1 site).
• Matches the convention used after the boost::ends_with removal in refactor: replace boost::ends_with with absl::EndsWith #4107. No behavior change.

Test plan

• Existing unit/integration tests pass in CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.31%. Comparing base (1d5edd3) to head (96f3763).

Additional details and impacted files

@@            Coverage Diff             @@
##               main    #4111    +/-   ##
==========================================
  Coverage     75.31%   75.31%            
  Complexity      711      711            
==========================================
  Files           763      763            
  Lines        143738   143738            
  Branches       1537     2074   +537     
==========================================
+ Hits         108252   108254     +2     
+ Misses        35183    35181     -2     
  Partials        303      303            

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