[conor-iqm]

POST /api/v3/ua/login is deprecated. Removed its "### Login" section and sidebar entry from guidelines/user-api.mdx entirely.

Across 13 quickstart guides and tutorials, each "Log in" step's header and curl/JSON examples already correctly showed /ua/oauth/token, but left two kinds of artifacts from before that switch:

• Top-of-page endpoint summary lists still linking to /ua/login (contextual-audience-quickstart, create-a-campaign-quickstart, conversion-quickstart, bid-model-quickstart).
• A stale TypeScript "Login" tab in every file's "Log in" step, still posting JSON to /ua/login with grantType/email fields - left over from before the endpoint became form-urlencoded oauth/token. Removed these (matching the established convention for oauth/token calls: no TypeScript tab anywhere else, including authentication-quickstart- guide.mdx's own "Refresh token" section and user-api.mdx's "OAuth token" section, since representing a form-urlencoded request as a JSON axios call would be wrong either way) and replaced the accompanying JSON request example with the same curl pattern already used elsewhere, fixing the field names along the way (grantType -> grant_type, email -> username, matching the real form param names).

Also fixed the same grantType/email artifacts in
authentication-quickstart-guide.mdx's own Log in and Refresh token request schema tables, which had survived independently of the stale TS tab.