docs: Fix deprecation comments for http package

[alexandear]

Summary

Fix deprecation comments for http, so the package will be automatically marked as "Deprecated" on https://pkg.go.dev/github.com/stretchr/testify/http.

Changes

• Update comments in the http package.

Motivation

The official documentation has a special deprecation syntax. It adds yellow background and "Deprecated" tag to the deprecated functions, so the deprecated types and functions are more clearly visible to readers.

Without changes:

With changes in the PR:

How to take these screenshots

• Run pkgsite in testify repo.
• Open in browser http://localhost:8080/github.com/stretchr/[email protected]/http.