Fix to_json escape

[webb-ben]

Overview

Fix to_json escape of <> to align the implementation with XML escape & unescape (source code)

Related Issue / discussion

Related to #2304, we have a process that outputs JSON keys of XML sitemaps. The sitemaps are unable to be read because to_json is incorrectly escaping <> by omiting the trailing semi-colon.

Additional information

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines