Updates for julia 1.8

[lassepe]

In preparation to make this package work with Julia 1.8 I made a bunch of changes

• Remove the Manifest.toml as it is version specific (and prevents the CI from testing with the latest versions)
• Explicitly convert the sum result to Cint since we previously accidentally relied on a bug in sum which has now been fixed

Related but not strictly necessary for 1.8 compatibility:

• Updated the CI.yml to always test on the latest stable version (rather than pinning to 1.7.0-rc1) and the nightly build (to catch issues like this earlier)
• Made Test a test dependency rather than a primary dependency of the package
• Removed the caching step from the CI because it's not necessary for the lightweight dependencies that we have and I have observed issues in the past (for another package) where this kind of caching caused trouble when the CI was updated to a new Julia version

I have also bumped the version to 0.3.6 which we should tag once this PR has been merged.

[codecov-commenter]

Codecov Report

Merging #9 (98719ba) into main (b264ee2) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   98.49%   98.66%   +0.17%     
==========================================
  Files           1        1              
  Lines         199      225      +26     
==========================================
+ Hits          196      222      +26     
  Misses          3        3              

Flag	Coverage Δ
unittests	98.66% <100.00%> (+0.17%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/nash.jl	98.66% <100.00%> (+0.17%)	⬆️

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