Draft: Migrate to OpenAPI 3.1

[philvarner]

Related Issue(s):

• OpenAPI 3.1 #31
• Conformance classes in OpenAPI schema #361

Proposed Changes:

1. Update schemas to OpenAPI 3.1
2. Clarify that the openapi30 conformance class from OAFeat shouldn't be used with these and remove from example landing pages
3. add x-conformance-classes extension property to info section

PR Checklist:

• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[m-mohr]

I know I opened #31, but as OGC API seems still to use 3.0, I'm wondering whether we should stick to 3.0 for now to now mix versions and have less issues with tooling? I think 3.1 support in tooling is still evolving and we also don't really make use of any 3.1 improvements yet (i.e. JSON Schema instead of "OpenAPI schema") so there's no urgent reason to migrate, right?

The example -> examples change seems unrelated to 3.0 -> 3.1 and it also looks like examples is actually meant to be a Map, but I need to check that more closely....

[philvarner]

Okay, I put this into draft and created a new PR for just the x-conformance-classes addition #385