Prior to this commit, the `antora` Gradle task silently failed to build
the reference documentation, since Antora uses the latest LTS release
for Node.js by default, and the latest LTS apparently does not work for
us.
- Use package.json so dependabot can automatically update JS dependencies
- Use @springio/antora-extensions (automatically apply default extensions
in proper order)
- Leverage set-algolia-env-extension to manage algolia env variables
- Update to latest ui
Closes gh-32863
Sam Brannen
Rob Winch
Sébastien Deleuze