Request: update README #136

Part of #103. Line numbers refer to branch chore/php-8.4-dependabot (#98). Verified = reproduced, suspected = from reading the code.

README.request-trait.md is stale:

• It says to implement build(), which is final in the trait.
• The examples have syntax errors (=> return ...).
• It only documents the deprecated findUser(), not loginUser(), sessionValue(), generateCsrfToken(), file uploads or DataBuilder.