update icon: openapi (original, original-wordmark, plain, plain-wordmark, line, line-wordmark)

[canaleal]

Double check these details before you open a PR

• PR does not match another non-stale PR currently opened
• PR name matches the format new icon: Icon name (versions separated by comma). More details here
• PR's base is the develop branch.
• Your icons are inside a folder as seen here
• SVG matches the standards laid out here
• A new object is added in the devicon.json file as seen here

This PR is related to #1998

Link to prove your SVG is correct and up-to-date.

• OpenAPI website
• OpenAPI Github

Created new plain and line SVGs created as the original SVGs have multiple paths and fills.
Used the color #91d400 for the font SVGs as the old color was not used in any of the SVGs.

[Snailedlt]

All parts of the line versions should be line-art, so this should be line-art too imo:

[lunatic-fox]

Great job! 👍🏼
Thank you for this contribution! 🚀