default semver constraint for empty string

[zywillc]

This PR is to fix 2 issues:

1. current tags filtering behavior will skip semver constraint for prereleases versions (although they are valid semver versions) when there is no explicit constraint parsed from image tags (i.e. empty string) which is inconsistent with the document description.
   https://github.com/argoproj-labs/argocd-image-updater/blob/master/pkg/image/version.go#L135-L147

2. some complex prerelease constraint rules are actually not supported untilsemver > 3.2.0

[codecov-commenter]

Codecov Report

Merging #552 (4c26a37) into master (49825ec) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   65.76%   65.80%   +0.03%     
==========================================
  Files          21       21              
  Lines        2039     2041       +2     
==========================================
+ Hits         1341     1343       +2     
  Misses        569      569              
  Partials      129      129              

Impacted Files	Coverage Δ
pkg/tag/tag.go	82.53% <ø> (ø)
pkg/image/version.go	42.04% <100.00%> (+1.34%)	⬆️

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[jannfis]

Thanks for your contribution and apologies that it took so long to come back to this PR.

Would you mind to rebase this PR (some of the lint changes have already been merged through another PR), and to supply a unit test for the * case?

Thanks a lot!