docs: refresh the v4 README for the maintenance line (#3202)

* docs: refresh the v4 README for the maintenance line

State that v4 is the maintenance line and link the v4 API reference and the
v5 upgrade guide. Add a Support section with the support dates, the Go 1.26
requirement and Echo Support. Move upgrade notes to the top, link the client
IP advisories, and add the v4.16.0 request-scheme change. Fix the example's
missing errors import, point badges at the v4 branch, drop the retired Go
Report Card badge, and update moved links.

* docs: match v4 README credits to master

* docs: list community and commercial support in the v4 README header

Author: vishr