GitLab: Use the correct visibility_level value for public repos

[ocean90]

Description If a project is public the value is 20, 0 means private. See https://docs.gitlab.com/ee/development/permissions.html#general-permissions

Closes #217.

How has this been tested?

Screenshots If applicable, add screenshots to show the visual change.

Types of changes

Checklist:

• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

[codecov[bot]]

Codecov Report

Merging #216 (12f46f9) into master (6407cac) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #216   +/-   ##
=========================================
  Coverage     81.59%   81.59%           
  Complexity      369      369           
=========================================
  Files            24       24           
  Lines           913      913           
=========================================
  Hits            745      745           
  Misses          168      168           

Impacted Files	Coverage Δ
inc/WebhookHandler/GitLab.php	100.00% <100.00%> (ø)

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