fix: support boost-asio 1.87.0

[miyanyan]

asio removed boost::asio::socket_base::max_connections, see https://www.boost.org/doc/libs/1_87_0/doc/html/boost_asio/history.html

[dbolduc]

/gcbrun

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.89%. Comparing base (3df97eb) to head (f44fba2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #436   +/-   ##
=======================================
  Coverage   85.89%   85.89%           
=======================================
  Files          96       96           
  Lines        2814     2814           
  Branches      203      203           
=======================================
  Hits         2417     2417           
  Misses        282      282           
  Partials      115      115           

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[miyanyan]

20/26 Test #20: site_test .................................***Timeout  60.06 sec
Running main() from gmock_main.cc
[==========] Running 26 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 26 tests from ExamplesSiteTest
[ RUN      ] ExamplesSiteTest.BearerToken
{"message":"standard C++ exception thrown by the function: TARGET_URL environment variable is not defined","severity":"error"}
{"message":"standard C++ exception thrown by the function: Error fetching credentials: INVALID_ARGUMENT: Cannot open PKCS#12 file (/dev/null):  error_info={reason=INVALID_ARGUMENT, domain=gcloud-cpp, metadata={gcloud-cpp.source.function=ParseServiceAccountP12File, gcloud-cpp.source.line=60, gcloud-cpp.source.filename=/home/runner/work/_temp/vcpkg/buildtrees/google-cloud-cpp/src/v2.29.0-a4f74cf33e.clean/google/cloud/internal/openssl/parse_service_account_p12_file.cc, gcloud-cpp.version=v2.29.0}}","severity":"error"}
[       OK ] ExamplesSiteTest.BearerToken (1[52](https://github.com/GoogleCloudPlatform/functions-framework-cpp/actions/runs/12828493249/job/35775932867?pr=436#step:10:53) ms)
[ RUN      ] ExamplesSiteTest.ConceptsAfterResponse
[       OK ] ExamplesSiteTest.ConceptsAfterResponse (0 ms)
[ RUN      ] ExamplesSiteTest.ConceptsAfterTimeout
Function running...
Function completed!
[       OK ] ExamplesSiteTest.ConceptsAfterTimeout (0 ms)
[ RUN      ] ExamplesSiteTest.ConceptsFilesystem
[       OK ] ExamplesSiteTest.ConceptsFilesystem (0 ms)
[ RUN      ] ExamplesSiteTest.ConceptsRequest

no idea why ExamplesSiteTest.ConceptsRequest timeout...