Bump github.com/pb33f/libopenapi from 0.21.8 to 0.21.9

[dependabot[bot]]

Bumps github.com/pb33f/libopenapi from 0.21.8 to 0.21.9.

Release notes

Sourced from github.com/pb33f/libopenapi's releases.

v0.21.9

Fixes a number of issues across models and the what_changed module after stress testing.

Also fixes the following issues:

• #380
• #400
• #403

Includes contributions from @​vishalg0wda @​jvogit @​vkd

Commits

• 5f6350b fixed build
• 2df8270 bump coverage
• 29a1bb4 bumping coverage
• 0aec29c fixed build issue
• 3c83b2e bump coverage
• 4f7718e bumped coverage
• 9ee3c09 fixed test, added nil check.
• 95a3798 Bumped coverage and fixed some glitches
• e4d33e3 addressed issue #403
• e81bfae fixed what-changed issue
• Additional commits viewable in compare view

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