Remove ActsAsTaggableOn

What does this MR do and why?

It replaces ActsAsTaggableOn gem with a simpler implementation introduced over several merge requests:

• Introduce Ci::Tag model (!159886 - merged)
• Introduce Ci::Taggings model (!159898 - merged)
• Introduce tags list and parser (!159908 - merged)
• Implement taggable concern (!160226 - merged)
• Use Ci::Tag model for autocomplete search (!161788 - merged)
• Remove ActsAsTaggableOn from DAST profiles (!161876 - merged)

After this is merged we can start working on writing the taggings records to their shard tables.

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Related to #467194 (closed)