Unhide XXenableManifestGeneration for skaffold init, remove unnecessary print line

[MarlonGamez]

Description
This PR unhides and renames the flag for manifest generation when using skaffold init. It's been changed from --XXenableManifestGeneration to --generate-manifests. I think this feature is at a point where we can put it into the hands of the user and try to get some feedback on it.

I've also removed an unnecessary line that is being printed in some cases when running init. I don't think it provides any value to the user. Please let me know if you think we should keep it there.

There's also a small fix to validation function that validates user input for port forwarding info.

User facing changes (remove if N/A)
Users will now see the option if they run skaffold init --help

[codecov]

Codecov Report

Merging #5152 (7e4a9bb) into master (6dbe2d2) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #5152      +/-   ##
==========================================
- Coverage   72.35%   72.32%   -0.04%     
==========================================
  Files         382      382              
  Lines       13589    13588       -1     
==========================================
- Hits         9832     9827       -5     
- Misses       3038     3040       +2     
- Partials      719      721       +2     

Impacted Files	Coverage Δ
pkg/skaffold/initializer/build/util.go	100.00% <ø> (ø)
pkg/skaffold/initializer/prompt/prompt.go	66.66% <0.00%> (ø)
cmd/skaffold/app/cmd/init.go	100.00% <100.00%> (ø)
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️

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