bump to bazel 9.1

[j2kun]

No description provided.

[j2kun]

Update: currently blocked by or-tools.

In file included from external/or-tools+/ortools/math_opt/cpp/message_callback.cc:29:                                                                                                                                   
In file included from external/or-tools+/ortools/base/logging.h:27:                                                                                                                                                     
external/abseil-cpp+/absl/status/status.h:666:40: error: reference to 'SourceLocation' is ambiguous                                                                                                                     
  666 |       absl::SourceLocation loc = absl::SourceLocation::current()) const& {                                                                                                                                      
      |                                  ~~~~~~^                                                                                                                                                                        
external/abseil-cpp+/absl/types/source_location.h:63:7: note: candidate found by name lookup is 'absl::lts_20260526::SourceLocation'                                                                                    
   63 | using SourceLocation = std::source_location;                                                                                                                                                                    
      |       ^                                                                                                                                                                                                         
external/or-tools+/ortools/base/source_location.h:39:7: note: candidate found by name lookup is 'absl::SourceLocation'                                                                                                  
   39 | class SourceLocation {                                                                                                                                                                                          
      |       ^                 

Looks like google/or-tools#5236 fixed it and we just have to wait for an or-tools release (or else point our dep directly to or-tool's main)

[j2kun]

Update, looks like Sep 15 is the expected date for v10: https://github.com/google/or-tools/milestone/21