change type from json to jsonschema in ../collection/queryables

[MathewNWSH]

Related Issue(s):
as above. Now every queryables endpoint shares the same type, which is:
"application/schema+json"

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable
• Changes are added to the changelog

[jonhealy1]

thanks @MathewNWSH. Can you add an entry in the Unreleased section of the Changelog.

[MathewNWSH]

Hey @jonhealy1 done :)

[jonhealy1]

Thanks @MathewNWSH!