[implot] Update to 0.15

[RT2Code]

Update implot port from 0.14 to 0.15 : https://github.com/epezent/implot/releases/tag/v0.15

• Changes comply with the maintainer guide
• SHA512s are updated for each updated download
• The "supports" clause reflects platforms that may be fixed by this new version
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.

[MonicaLiu0311]

The usage test passed (header files found):

implot provides CMake targets:

    # this is heuristically generated, and may not be correct
    find_package(implot CONFIG REQUIRED)
    target_link_libraries(main PRIVATE implot::implot)

[BillyONeal]

Thanks for the update!