fix(develop): Use user.name instead of user.username (#15628)

https://github.com/getsentry/sentry-conventions/blob/main/generated/attributes/user.md

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Author: sentrivana

develop-docs/sdk/telemetry/spans/span-protocol.mdx

@@ -183,7 +183,7 @@ Empty attributes must be omitted.
 | `user.id` | string | The user ID (gated by `sendDefaultPii`) |
 | `user.email` | string | The user email (gated by `sendDefaultPii`) |
 | `user.ip_address` | string | The user IP address (gated by `sendDefaultPii`) |
-| `user.username` | string | The user username (gated by `sendDefaultPii`) |
+| `user.name` | string | The user username (gated by `sendDefaultPii`) |
 | `thread.id` | string | The thread ID |
 | `thread.name` | string | The thread name |
 | `sentry.sdk.name` | string | Name of the Sentry SDK (e.g., "sentry.php", "sentry.javascript") |