Add line search based on MM, require Julia 1.8

[JeffFessler]

Perhaps this belongs elsewhere, e.g., LineSearches.jl, or in some "ImageReconOptimizers" but for now this is a convenient place for me to put it.

To reduce carbon footprint, no longer test Julia 1.6.

[codecov]

Codecov Report

Base: 100.00% // Head: 99.92% // Decreases project coverage by -0.07% ⚠️

Coverage data is based on head (41d9df3) compared to base (a2dcd5e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #126      +/-   ##
===========================================
- Coverage   100.00%   99.92%   -0.08%     
===========================================
  Files           37       39       +2     
  Lines         1284     1372      +88     
===========================================
+ Hits          1284     1371      +87     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
src/utility/downsample.jl	100.00% <ø> (ø)
src/algorithm/general/dot-curv.jl	100.00% <100.00%> (ø)
src/algorithm/general/dot-grad.jl	100.00% <100.00%> (ø)
src/algorithm/general/ls-mm.jl	100.00% <100.00%> (ø)
src/algorithm/general/ncg.jl	100.00% <100.00%> (ø)
src/io/shows.jl	100.00% <100.00%> (ø)
src/algorithm/general/ogm_ls.jl	98.11% <0.00%> (-1.89%)	⬇️
src/MIRT.jl

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