Add maven central staging repo to allowed list

This repo is sometimes listed in publish scripts. We don't need to bail out when we see it.

• Reference: https://central.sonatype.org/publish/publish-guide/
• Reference: https://git.zx2c4.com/wireguard-android/tree/tunnel/publish.gradle?id=8152191277ca0fb740a051e99c482c9b90c65a8b#n50