Revert OpenAPI version number to 3.0.0

This was bumped in 8be2ec4 but we do not actually abide by 3.1.0, as can be seen in the use of `example` instead of `examples. v3.1.0 is a breaking change, the technical committee decided not to follow semver, see <https://github.com/OAI/OpenAPI-Specification/releases/tag/3.1.0-rc0>.
Support for OpenAPI v3.1 is not offered by the current Legislation Explorer version and upgrading is too costly. See <openfisca/legislation-explorer#236>.

openfisca_web_api/openAPI.yml

@@ -1,4 +1,4 @@
-openapi: "3.1.0"
+openapi: "3.0.0"
 
 info:
   title: "{COUNTRY_PACKAGE_NAME} Web API"
@@ -137,8 +137,8 @@ components:
       type: "object"
       additionalProperties:
         $ref: "#/components/schemas/Value"
-      propertyNames:  # this keyword is part of JSON Schema but is not supported in OpenAPI Specification at the time of writing, see https://swagger.io/docs/specification/data-models/keywords/#unsupported
-        pattern: "^[12][0-9]{3}-[01][0-9]-[0-3][0-9]$"  # all keys are ISO dates
+      # propertyNames:  # this keyword is part of JSON Schema but is not supported in OpenAPI v3.0.0
+      # pattern: "^[12][0-9]{3}-[01][0-9]-[0-3][0-9]$"  # all keys are ISO dates
 
     Value:
       oneOf: