Allow updates.limit option to be zero

[fthomas]

This allows the repo config option updates.limit to be zero so that it
can now be zero or positive instead of positive only. A zero
updates.limit means that Scala Steward will not create or update PRs.
It can be used to temporarily disable Scala Steward in a repo or if it
is used in the default repo config it would be similar to a hypothetical
dry-run option.

[codecov]

Codecov Report

Merging #1828 (1d4f890) into master (055819b) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1828      +/-   ##
==========================================
+ Coverage   74.87%   74.89%   +0.01%     
==========================================
  Files         123      123              
  Lines        2078     2079       +1     
  Branches       53       57       +4     
==========================================
+ Hits         1556     1557       +1     
  Misses        522      522              

Impacted Files	Coverage Δ
...ala/org/scalasteward/core/nurture/NurtureAlg.scala	4.90% <100.00%> (ø)
...g/scalasteward/core/repoconfig/UpdatesConfig.scala	100.00% <100.00%> (ø)
...ain/scala/org/scalasteward/core/util/package.scala	100.00% <100.00%> (ø)

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