Update middleware upgrade guide w/ more rationale (#38074)
Update middleware upgrade guide w/ more rationale (#38074)

* Update middleware upgrade guide w/ more rationale

The API changes for GA are general improvements.

Our middleware upgrade guide is very helpful about describing the changes
that were made and how to react to them; include some additional "why" to
rationalize these changes.

* Update middleware-upgrade-guide.md

Avoid words like "simple" and "easy"

* Update middleware-upgrade-guide.md

Clarify the issues around "security" with responses - the previous verbiage made it sound like there was a security vulnerability; there was not.

* Apply suggestions from code review

Co-authored-by: Lee Robinson <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@leerob.io>

* Apply suggestions from code review

* Update errors/middleware-upgrade-guide.md

* Update errors/middleware-upgrade-guide.md

Co-authored-by: Balázs Orbán <59bd0a3ff43b32849b319e645d4798d8a5d1e889@balazsorban.com>

* Apply suggestions from code review

* Update canary -> latest

* Apply suggestions from code review

Co-authored-by: Lee Robinson <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@leerob.io>
Co-authored-by: Balázs Orbán <59bd0a3ff43b32849b319e645d4798d8a5d1e889@balazsorban.com>
Co-authored-by: JJ Kasper <7323a5431d1c31072983a6a5bf23745b655ddf59@jjsweb.site>
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