stdcoroutine-4: Remove Boost.Coroutine dependency and old Coro API

[pratikmankawde]

High Level Overview of Change

Delete the old Boost.Coroutine-based Coro class, postCoro() API, and the entire Coro.ipp implementation file. Remove Boost.Coroutine and Boost.Context from all build dependencies (CMake, Conan).

Context of Change

This is Milestone 4 (final) of the Boost.Coroutine → C++20 coroutines migration:

• Milestone 1 (pratik/std-coro/add-coroutine-primitives): Added CoroTask<T>, CoroTaskRunner, JobQueueAwaiter, postCoroTask()
• Milestone 2 (pratik/std-coro/migrate-entry-points): Migrated all production entry points
• Milestone 3 (pratik/std-coro/migrate-test-code): Migrated all test code
• Milestone 4 (this PR): Delete old code and build dependencies

After this change, zero references to postCoro, JobQueue::Coro, Coro.ipp, or Boost.Coroutine remain in the codebase (only the migration plan doc references them).

Type of Change

• Refactor (non-breaking change that only restructures code)
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)

API Impact

• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)

Before / After

Build dependencies removed:

• Boost::coroutine from cmake/deps/Boost.cmake (COMPONENTS and target_link_libraries)
• Boost::context from cmake/XrplConfig.cmake (was only needed by Boost.Coroutine)
• BOOST_COROUTINES_NO_DEPRECATION_WARNING from cmake/XrplInterface.cmake
• boost::coroutine from conanfile.py

Code removed:

• include/xrpl/core/Coro.ipp (122 lines, entire file deleted)
• class Coro, Coro_create_t, postCoro() from JobQueue.h (~180 lines including comments)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.8%. Comparing base (91daa1f) to head (f4ed00e).

Additional details and impacted files

@@                        Coverage Diff                        @@
##           pratik/std-coro/migrate-test-code   #6429   +/-   ##
=================================================================
  Coverage                               79.8%   79.8%           
=================================================================
  Files                                    850     850           
  Lines                                  67854   67854           
  Branches                                7562    7558    -4     
=================================================================
+ Hits                                   54135   54144    +9     
+ Misses                                 13719   13710    -9     

Files with missing lines	Coverage Δ
include/xrpl/core/JobQueue.h	90.9% <ø> (ø)

... and 3 files with indirect coverage changes

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