Drop support for Julia 1.5 and older

[rofinn]

This seemed like the easier options since Documenter dropped these Julia version a couple weeks ago and is now causing compat issues.

https://github.com/invenia/FeatureTransforms.jl/actions/runs/2786559009

[codecov]

Codecov Report

Merging #110 (3b81125) into main (4a636c8) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
- Coverage   98.90%   98.87%   -0.04%     
==========================================
  Files          15       14       -1     
  Lines         183      178       -5     
==========================================
- Hits          181      176       -5     
  Misses          2        2              

Impacted Files	Coverage Δ
src/apply.jl	100.00% <0.00%> (ø)
src/periodic.jl	100.00% <0.00%> (ø)
src/one_hot_encoding.jl	100.00% <0.00%> (ø)
src/FeatureTransforms.jl

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[rofinn]

Looks like coverage reporting might be different across Julia releases? Also, I think the Julia 1.5 jobs is stuck cause it's required on main? I believe that should go away once merged?

[Wynand]

This seems to work when the Julia 1.5 test isn't included: https://github.com/invenia/FeatureTransforms.jl/actions/runs/2791698474

And coverage seems to only be an issue because there are 5 less lines of code: https://github.com/invenia/FeatureTransforms.jl/pull/110/checks?check_run_id=7658996965

[mzgubic]

I'm fine with dropping support for 1.5.

[AlexRobson]

Small question - Is this actually a non-breaking release?

I had done something similar for 0.3 for NamedDims and someone commented that it's a minor release.

invenia/NamedDims.jl#202 (comment)

(I've got to here because I'm trying to see which packages need NamedDims 1 and I also encountered this package due to a resolution issue from one of our internal packages)

[mzgubic]

Small question - Is this actually a non-breaking release?

Yes, indeed it is, sorry if it caused trouble. I imagine it can be resolved by bumping compat in a few places?