Fix KernelFunctions on Julia 1.3

[devmotion]

https://github.com/JuliaGaussianProcesses/KernelFunctions.jl/pull/531 (included in KernelFunctions 0.10.62 and 0.10.63) broke KernelFunctions on Julia 1.3, which broke support and tests in downstream packages such as AbstractGPsMakie. The problem was that in CI the oldest supported Julia version was not tested.

I think it would be fine to drop support for Julia < 1.6 (even though I don't see the need as long as it works and does not impact development significantly) but before changing the Julia compat of KernelFunctions we have to fix support on Julia >= 1.3: Otherwise Pkg will always give people on such older Julia versions a broken KernelFunctions version (that currently can't even be loaded). Alternatively, we could mess with the registry and try to fix compat bounds retroactively but in my experience it's generally considered better practice to register bug fix releases than modifying existing releases (e.g. yanking a release can also break existing Manifest.toml files).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.88%. Comparing base (2a6e5e9) to head (ee83792).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #554   +/-   ##
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  Coverage   90.88%   90.88%           
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  Files          52       52           
  Lines        1459     1459           
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  Hits         1326     1326           
  Misses        133      133           

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