CompatHelper: bump compat for "NFFT" to "0.6"

[github-actions]

This pull request changes the compat entry for the NFFT package from 0.5 to 0.5, 0.6.

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.

[codecov]

Codecov Report

Merging #82 (361817c) into master (7cffe83) will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   99.87%   99.75%   -0.13%     
==========================================
  Files          49       49              
  Lines        2409     2409              
==========================================
- Hits         2406     2403       -3     
- Misses          3        6       +3     

Impacted Files	Coverage Δ
src/nufft/nufft.jl	100.00% <100.00%> (ø)
src/fbp/sino_geom.jl	97.82% <0.00%> (-2.18%)	⬇️

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[JeffFessler]

Also includes changes within to handle breaking changes in NFFT 0.6.