Prep for v7.0 and refactor openMP build flags

[r-devulap]

Refactor the build system to improve OpenMP support and updates the project version. The most significant changes include replacing manual OpenMP flag handling with Meson's dependency mechanism, updating the project version to 7.0.x, and ensuring consistent dependency declarations across the build files.

Build system improvements:

• Replaced manual OpenMP flag handling (openmpflags) with Meson's dependency('openmp') and declare_dependency mechanism. This change centralizes OpenMP configuration and simplifies its usage throughout the build files. (meson.build, lib/meson.build, tests/meson.build) [1] [2] [3]
• Updated all library and test targets to use the new omp_dep dependency instead of manually appending OpenMP flags. (meson.build, lib/meson.build, tests/meson.build) [1] [2] [3] [4] [5] [6]

Version updates:

• Updated the project version from 6.0.x to 7.0.x to reflect significant changes in the build system. (meson.build)
• Updated the pkgconfig version from 4.0 to 7.0 to align with the new project version. (meson.build)

[sterrettm2]

Looks good to me! Seems like a very reasonable refactor.