3.2: allow openapi field to just specify MAJOR.MINOR and omit the PATCH number

[ralfhandl]

Requiring openapi: MAJOR.MINOR.PATCH and then stating in the spec that PATCH actually is meaningless is confusing.

Better allow (or require?) to write openapi: MAJOR.MINOR.

[handrews]

We could relax this requirement and allow 3.2 (etc.) but we can't outright forbid the patch version due to compatibility issues. At least not for 3.1.x and 3.0.x. I suppose we coudl say "X.Y.Z" if Y <=1, "X.Y" if Y>=2.