Allow comma-separated labels for issue update

[profclems]

Description
Use StringSliceP instead of StringArrayP to support both comma-separated values
and multiple usages.
https://pkg.go.dev/github.com/spf13/pflag@v1.0.5#StringArray
https://pkg.go.dev/github.com/spf13/pflag@v1.0.5#StringSlice

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)

[codecov]

Codecov Report

Merging #477 (e614cdc) into trunk (bf5611e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            trunk     #477   +/-   ##
=======================================
  Coverage   51.45%   51.45%           
=======================================
  Files          83       83           
  Lines        5455     5455           
=======================================
  Hits         2807     2807           
  Misses       2319     2319           
  Partials      329      329           

Impacted Files	Coverage Δ
commands/issue/update/issue_update.go	64.47% <100.00%> (ø)

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