Incorrect API version endpoints
Verified in v2.6.1
The endpoint to `API Versions` resource should be `/api_versions` and not `/api_version`.
See https://www.etsi.org/deliver/etsi_gs/NFV-SOL/001_099/013/02.06.01_60/gs_nfv-sol013v020601p.pdf, section 9.3.2
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