Deprecate ecbuild_find_(mpi|omp).cmake macros

[marcosbento]

Description

These changes deprecate ecbuild_find_(mpi|omp).cmake macros, in favour of directly using FindMPI and FindOpenMP -- as suggested by #126.

Contributor Declaration

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• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[Copilot]

Pull request overview

This PR marks the legacy ecbuild_find_mpi/ecbuild_find_omp-related macros as deprecated in favor of using CMake’s built-in FindMPI and FindOpenMP, aligning with Issue #126 and the broader move away from ecbuild-specific find wrappers.

Changes:

• Updated the RST-style documentation blocks in ecbuild_find_mpi.cmake and ecbuild_find_omp.cmake to clearly label the affected macros as deprecated and direct users to FindMPI/FindOpenMP.
• Added runtime deprecation warnings inside the MPI-related macros (ecbuild_find_mpi, ecbuild_enable_mpi, ecbuild_include_mpi) via a new helper call at macro entry.
• Added runtime deprecation warnings inside the OpenMP-related macros (ecbuild_find_omp, ecbuild_enable_omp, ecbuild_enable_ompstubs) via a new helper call at macro entry.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
cmake/ecbuild_find_omp.cmake	Adds documentation and runtime deprecation for ecbuild_find_omp, ecbuild_enable_omp, and ecbuild_enable_ompstubs to steer users toward CMake’s FindOpenMP.
cmake/ecbuild_find_mpi.cmake	Adds documentation and runtime deprecation for ecbuild_find_mpi, ecbuild_enable_mpi, and ecbuild_include_mpi to steer users toward CMake’s FindMPI.

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[wdeconinck]

Thanks @marcosbento for pushing this further!