CompatHelper: bump compat for CxxWrap to 0.15, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the CxxWrap package from 0.14 to 0.14, 0.15. This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

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From https://github.com/JuliaIPU/IPUToolkit.jl/pull/35#pullrequestreview-1932658564:

libcxxwrap_julia_jll 0.11 isn't compatible with CxxWrap 0.15, so this isn't useful at the moment