Point azure-data-schemaregistry-apacheavro:1.2.0-beta.2 to Maven Central

## Problem The Java docs CI build aborts on `com.azure:azure-data-schemaregistry-apacheavro:1.2.0-beta.2`. Its `.jar` and `.pom` are cached on the azure-sdk public feed (HTTP 200), but the **`-sources.jar` is not**, so an anonymous download triggers the upstream-save auth gate and returns **HTTP 401**: ``` HTTP 401 for .../azure-data-schemaregistry-apacheavro/1.2.0-beta.2/ azure-data-schemaregistry-apacheavro-1.2.0-beta.2-sources.jar → Java2docfx exited with errors. ``` Because `java2docfx` fails the whole run on the first failed package, this one missing sources jar fails the entire docs build (observed in apidrop Content CI build 20260626.2 / id 614443). ## Change Repoint this single preview package to **Maven Central**, where all three artifacts (`.jar`, `-sources.jar`, `.pom`) are available anonymously (published 2023-02-13). This matches the track-1 fix in #2655 and the existing `client-sdk.json` precedent. - `metadata/preview/azure-data-schemaregistry-apacheavro.json` — add `DocsCiConfigProperties.packageDownloadUrl` = `https://repo1.maven.org/maven2`. - `package.json` — set the generated preview entry's `packageDownloadUrl` to Maven Central (kept in sync so the docs CI build picks it up before the next docindex regeneration). ## Validation - A full anonymous gap scan of all 695 packages in `package.json` (per-file: jar/sources.jar/pom, honoring each package's configured download URL) found this as the **only** remaining gap after #2655 merged. - All three Maven Central artifacts confirmed downloadable (HTTP 206 on ranged GET). - A docs CI build (apidrop def 3188) is being queued against this branch to confirm it goes green. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>