Move deprecated Vararg UnionAll outside

[oxinabox]

Closes #492.
This should fix julia 1.7

[codecov-commenter]

Codecov Report

Merging #493 (7255e78) into main (38c4089) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #493   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files          15       15           
  Lines         822      822           
=======================================
  Hits          764      764           
  Misses         58       58           

Impacted Files	Coverage Δ
src/projection.jl	97.47% <ø> (ø)

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[mcabbott]

This is narrower than before, although I doubt it matters:

julia> _PZ_Tuple = Tuple{_PZ,Vararg{<:_PZ}}; # old

julia> (ProjectTo{NoTangent}(), ProjectTo{ZeroTangent}()) isa _PZ_Tuple
true

julia> _PZ_Tuple2 = Tuple{PZ,Vararg{PZ}} where PZ<:_PZ # new
Tuple{PZ, Vararg{PZ}} where PZ<:(ProjectTo{<:AbstractZero})

julia> (ProjectTo{NoTangent}(), ProjectTo{ZeroTangent}()) isa _PZ_Tuple2
false

julia> ProjectTo(false)
ProjectTo{NoTangent}()

We could also consider ripping out all these <:AbstractZero and replacing them with ProjectTo{NoTangent} exactly. Or one step further, with Returns{NoTangent}.

[oxinabox]

We could also consider ripping out all these <:AbstractZero and replacing them with ProjectTo{NoTangent} exactly. Or one step further, with Returns{NoTangent}.

Maybe. Out of scope for this PR

[simeonschaub]

LGTM