Add XML, JSON, and YAML media types to `fedramp_values.xml`

[GaryGapinski]

Extended Description

• Add missing media types for XML and JSON

Acceptance Criteria

• All Schematron assertion messages are declarative statements which affirm the positive test outcome.
• All Schematron assertion diagnostic messages are declarative statements which explain the negative test outcome.
• The Schematron code has no assertion failures when validated using src/validations/styleguides/sch.sch using the basic phase.
• XSpec unit tests for positive and negative Schematron assertion outcomes accompany all Schematron assertions (where feasible).

Story Tasks

• Update the document src/content/resources/xml/fedramp_values.xml.
• Format and indent as initial, separate commit (document has mixed 3- and 4-character indents).
• Add additional media types to end of <value-set name="media-type"> allowed-values stanza.
• Add media type application/xml.
• Add media type text/xml (see RFC 7303 — both are allowed, application/xml is recommended).
• Add media type application/json.
• Add media type application/yaml (see draft-ietf-httpapi-yaml-mediatypes-03 which was recently in WG Last Call).
• Update the document metadata.

Definition of Done

• Acceptance criteria met
• Unit test coverage of our code > 95%
• Automated code quality checks passed
• Security reviewed and reported
• Reviewed against plain language guidelines
• Code must be self-documenting
• No local tech debt
• Load/performance tests passed – needs to be created/automated
• Documentation updated
• Architectural Decision Record completed as necessary for significant design choices
• PR reviewed & approved
• Source code merged