Filter issues with nonnegative integer weights

What does this MR do?

• updates default weight range from 1-9 to 0-20
• adds ability to define a custom weight in the issue search filter dropdown
• change "weight" issue search weight tag to "number"

GIF Demo

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Does this MR meet the acceptance criteria?

• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #3969 (closed)