[ui/utils/query_string]: Remove unused methods & migrate apps to querystring lib

[lukeelmers]

Part of #51560

Currently src/legacy/ui/public/utils/query_string is used several places, but according to this comment can probably be replaced in favor of the existing querystring dependency in Kibana.

• QueryString.encode -> replace uses with querystring.stringify (double-check outputs are identical)
• QueryString.decode -> replace uses with querystring.parse (double-check outputs are identical)
• QueryString.findInUrl -> can be removed as it appears to be unused
• QueryString.param -> a simple util that's only used in canvas. we should just copy it over
• QueryString.replaceParamInUrl -> only used in state management and will get left behind
• Update 7.7 dev docs issue with documentation on the above changes

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)