Fix no-invalid-media-type-examples in OpenAPI

What does this MR do and why?

This MR fixes 20 no-invalid-media-type-examples warnings in the OpenAPI v3 specification by updating invalid examples in lib/api/access_requests.rb and regenerating doc/api/openapi/openapi_v3.yaml.

The invalid examples were structured as a flat hash, but the schema requires a nested user object. By fixing this, 22 instances of the no-invalid-media-type-examples warnings are resolved.

Schema	Old Value	New Value
Entities::AccessRequester	id: 1\\ username: raymond_smith\\ name: Raymond Smith\\ state: active\\ created_at: '2012-10-22T14:13:35Z'\\ access_level: 20	user:\\ id: 1\\ username: raymond_smith\\ name: Raymond Smith\\ state: active\\ requested_at: '2012-10-22T14:13:35Z'
Entities::Member	id: 1\\ username: raymond_smith\\ name: Raymond Smith\\ state: active\\ created_at: '2012-10-22T14:13:35Z'\\ access_level: 20	user:\\ id: 1\\ username: raymond_smith\\ name: Raymond Smith\\ state: active\\ access_level: '20'\\ created_at: '2012-10-22T14:13:35Z'

How to set up and validate locally

Regenerate the OpenAPI docs:

 bin/rake gitlab:openapi:v2:generate
 bin/rake gitlab:openapi:v3:generate

Run the Redocly CLI to confirm output resolves all 22 warnings in files:

npx @redocly/cli lint doc/api/openapi/openapi_v3.yaml --extends=recommended --max-problems 10000 --format=json | jq -r '.problems[] | select(.ruleId == "no-invalid-media-type-examples")'

/cc @abignotti